Question

read the passage and then answer the question.
a 2015 study by nasa scientists revealed that up to one-fifth of marss surface may once
have been covered by water. the researchers obtained this figure by comparing the water
in marss atmosphere today with the water found inside a 4.5 billion-year-old martian
meteorite. specifically, the scientists calculated the ratio of h₂o to hdo in each water
sample. hdo is a variant of water containing deuterium, which is an isotope of
hydrogen. h₂o is light and easily escapes into space from marss atmosphere; hdo,
meanwhile, is heavier due to the addition of a neutron in deuterium and is more likely to
persist in the atmosphere as a result. the scientists used the difference between the two
water samples h₂o:hdo ratios to calculate how much water mars has lost in the last 4.5
billion years—an estimated eighty-seven percent.
what is the meaning of isotope as used in the passage?
a variant of water with a different number of
neutrons
a variant of an element found only in meteorites
a variant of an element with a different number of
neutrons
a variant of water found only in meteorites

Explanation:

The passage states that HDO is a variant of water containing deuterium, which is an isotope of hydrogen. Isotopes are variants of an element (in this case, hydrogen) that have the same number of protons but different numbers of neutrons. The text also mentions that HDO is heavier due to the addition of a neutron in deuterium. Option 1 is incorrect because it refers to water (a compound, not an element). Option 2 is wrong as isotopes are not "found only in meteorites." Option 4 is incorrect for the same reason as option 1 (referring to water instead of an element).

Answer:

a variant of an element with a different number of neutrons