As close friends, they used to walk to school .
- A.face to face
- B.little by little
- C.step by step
- D.side by side
答案
正确答案:D
知识点:介词短语

face to face面对面;little by little缓慢地,一点一点地;step by step逐步地;side by side并肩地。句意为“作为密友,他们过去常常肩并肩地走去学校”,可知答案选D。

略
