must/mustn't用法 — 英语词汇题
题目
看场景，用 must 或 mustn't 填空（每词限用一次，注意形式正确）。
1. The sign says "No littering." We ______ throw rubbish on the ground.
2. The sign says "No swimming." We ______ swim in this river.
3. In the library, we ______ keep quiet.
4. On the road, we ______ look left and right first.