Question

Mary liked to look back on her high—school days，     she thought were the happiest in her life．

1. A.
   that
2. B.
   what
3. C.
   which
4. D.
   when

Answer 1

C
本题考查定语从句。she thought作插入语，故先行词days在非限定性定语从句中作主语。故选C。