大全670，看了以前的讨论，还是不明白

Stars like the sun can continue to shine steadily for billions of years because its light and heat are produced by nuclear fusion, in which titanic releases of energy result in the loss of only tiny amounts of mass.

(A) its light and heat are produced by nuclear fusion, in which

(B) nuclear fusion produces its light and heat, in which

(C) nuclear fusion produces their light and heat, where

(D) its light and heat are produced by nuclear fusion, where

(E) their light and heat are produced by nuclear fusion, in which

答案为：A，不明白where,和in which 的用法区别，这里为什么不能用where?用in which是什么意思？