Tom succeeded in doing experiment,----was expected by his in

Tom succeeded in doing the experiment,__as__was expected by his instructor.
答案是AS,解释为which._as__was expected by his instructor是定语从句部分.BCD都是凑数的,用来迷惑你.
再问： 为什么可以看成是定语从句呢？
再答： 因为as指代tom成功这件事。