1. It was the first time that she ______ such a heavy rain.

A. had witnessed   B. has witnessed    C. should witness   D. witnessed

2 ________ in doing research on wild plants, he seldom returned home.

A. Have buried     B. To be buried C. Buried       D. Burying

3._______ time went by, the color of the wall faded out.

  A. When       B. While        C. As       D. Since

4. The government should pay attention to the problem _______ stay-at-home children are faced with.

   A. what      B. that     C. how     D. why