What is a limiting factor??

look below

Explanation:

A limiting factor is a variable of a system that causes a noticeable change in output or another measure of a type of system. The limiting factor is in a pyramid shape of organisms going up from the producers to consumers and so on. A factor not limiting over a certain domain of starting conditions may yet be limiting over another domain of starting conditions, including that of the factor.

the answer is a or when it says crossing over

satellites are frequently used to make observations of the orbit on earth