We believe that Earth is about 4.6 billion years old. At present we are forced to look at other bodies in the solar system to know what the early history of the Earth was like.
Studies of our moon, Mercury, Mars, and the large satellites of Jupiter and Saturn have provided ample evidence that all these large celestial bodies were bombarded by smaller objects in a wide variety of sizes shortly after the larger bodies had formed. This same bombardment must have affected Earth as well. The liner record indicates that the rate of impacts decreased to its present low level about 4 billion years ago. On Earth, subsequent erosion and crustal motions have obliterated the craters that must have formed during this epoch.
Scientists estimate the Earth's age by measuring the ratios of various radioactive elements in rocks. The oldest Earth rocks tasted so far are about 3 billion years old. But no one knows whether these are the oldest rocks on Earth. Tests on rocks from the moon and on meteorites show that these are about 4.6 billion years old. Scientists believe that this is the true age of solar system and probably the true age of the Earth.

1) In line 11, the word "obliterated" means :
A - created. B - destroyed.
c - changed. D - eroded

2) According to this passage, how do scientists estimate the age of the Earth?
A - By measuring the ratios of radioactive elements in rocks.
B - By examining fossils.
C - By examining volcanic activity.
D - By studying sun sports.

3) Scientists estimate the age of the Earth as
A - 3 billion years old.
B - 4 billion years old.
C - 4.6 billion years old.
D - 6 billion years old.

4) Which of the following processes led to the obliteration of the carts formed by the bomberdment of the Earth by cclcstial bodies?
A - Volcanic activity. B - Solar radiation.
C - Gravity. D - Crustal motions.

5) According to the passage, why are scientists forced to look at other bodies in the solar system to determine the Carly history of the Earth?
A - Human alteration of the Earth.
B - Erosion and crustal motions.
C - Solar flares.
D - Deforestation.

6) What is the Best title for this passage?
A - "Determining the Age of the Earth".
B - "Determining the Age of the Solar System"
C - "Erosion and Crustal Motion of Earth"
D - "Radioactive Elements in Rocks"

7) Which of the following bodies was not studied to give evidence that the Earth was bombarded in its early history?
A - Mars B - Mercury
C - Jupiter D - Earth's moon

8) Bombardment of the Earth at one time by various sized bodies is ..........
A - inferred from what happened on other planetary bodies.
B - documented fact.
C - proven by the lunar record.
D - indicated by erosion.

9) The level of impacts of the bombardments of the Earth have
A - decreased to below normal.
B - increased to a current high.
C - increased after a periodic low.
D - decreased to the current low level.

10) In the line 8, the word "bombardment" means
A - an avoidance B - an assault
C - an effect D - a cause

A snowfall consists of myriads of minute ice crystals that fall to the ground in the frozen precipitation. The formation of snow begins with these ice crystals in the subfreezing strata of the middle and upper atmosphere when there is an adequate line supply of moisture present. At the core of every ice crystal is a minuscule nucleolus, a solid particle of matter around which moisture condenses and freezes. Liquid water droplets floating in the supercooled atmosphere and free ice crystals cannot coexist within the ice crystals to rob the liquid droplets of their moisture and grow continuously. The process can be very rapid, quickly creating sizable ice crystals, some of which adhere to each other create a cluster of ice crystals or a snowflake. Simple flakes possess a variety of beautiful forms, usually hexagonal, through the symmetrical shapes reproduced in most microscope photography of snowflakes are not usually found in actual snowfalls. Typically, snowflakes in actual snowfalls consist of broken fragments and clusters of adhering ice crystals.
For a snowfall to continue once it starts, there must be a constant inflow of moisture to supply the nuclei. This moisture is supplied by the passage of an atmosphere. The pacific Ocean is the source of moisture for most snowfalls west of the Rockey Mountains, while the Gulf of Mexico and the Atlantic Ocean feed water vapor into the air currents over the central and eastern sections of the United States. Other geographical features also can be source of moisture for some snowstorms. For example, areas adjacent to the Great Lakes experience their own unique lake-effect storms, employing a variation of the process on a local scale. In addition, mountainous sections or rising terrain can initiate snowfalls by the geographical lifting of a moist air stream.