Subject _____ is the mechanism of high temperature supercond

Subject _____ is the mechanism of high temperature superconductivity.
[A] which continues to be intense debate.
[B] on which there continues to be intense debate
[C] on which it continues to be intense debate
[D] what continues to be intense debate
这题 选哪个?为什么?请翻译整句
为什么 清帮我分析一下并翻译整句