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
本題考查定語從句。she thought作插入語，故先行詞days在非限定性定語從句中作主語。故選C。