Which statement is true?
A) It is likely that collisions of comets brought water to ancient Earth.
B) Water is useful, but not essential, for life as we know it.
C) The first water on Earth was generated by metabolic activity of cyanobacteria.
D) As early Earth cooled and chemicals dissolved in the water, chemical reactions would not have occurred, because there was no oxygen.
E) There is no evidence that water has ever occurred on Mars.

Question

Quizplus · Accepted Answer

The theory of cometary bombardment suggests that water on Earth may have come from collisions with comets that contained water. This theory is supported by the fact that the composition of water on Earth matches that of comets.

Answer: B
 Water is necessary for life as we know it, but there may be other forms of life that do not require water.

Answer: C
 The metabolic activity of cyanobacteria can produce oxygen, but it does not generate water. The theory of cometary bombardment is more widely accepted as the source of water on Earth.

Answer: D
 Chemical reactions can occur in the absence of oxygen, and some of the first life forms on Earth did not require oxygen.

Answer: None
 Evidence from NASA's Mars missions suggests that water has existed on Mars in the past, and may still exist in the form of ice or underground reservoirs.

Which Statement Is True