The opposite reciprocal of 7/21 is

-21/7 or -3 (depends on your instruction)

Explanation:

Opposite: the opposite of the current sign

if it is currently negative [-], then its opposite is positive [+]If it is currently positive [+], then its opposite is negative [-]

Reciprocal: changing the place of numerator and denominator

The reciprocal of a/b is b/a

7/21

Opposite: since it is positive, the opposite is negative

Reciprocal: since it is 7/21, we change the place, and it is 21/7=3

Opposite reciprocal of 7/21 is -21/7 or -3

The correct answer is:   [b]:   "has no atmosphere" .

If a particular resource is not useful or needed then the price would be considerably low. once that resource is needed the price or value will raise depending on how useful or needed it is especially if the resource is low in quantity, the consumers would pay whatever they have to, to get that material.