Astar is thought to be born when
a. a protostar reaches a temperature high enough for nuclear fusion to begin.
b. a red giant collapses on itself and becomes a black hole.
c. pressure within a protostar becomes so great that a supernova occurs.
d. increasing heat within a supergiant causes gravity to be greater than pressure.

a. a protostar reaches a temperature high enough for nuclear fusion to begin.

Explanation:

81 is a composite number.

the answer would be the third option, "no, ariel should have added 9^2 and 12^2 and compared that to 15^2."

this is found through the pythagorean theorem, which is a^2 + b^2 = c^2.

this is the correct way of solving the problem:

9^2 + 12^2 = 15^2

81 + 144 = 225

225 = 225