问题补充：
We must keep our room clean. Dirt and disease go you know.
A. from time to time
B. hand in hand
C. step by step
D. one after another
答案：
A. from time to time
(答案→)B. hand in hand
C. step by step
D. one after another
解析：本题是关键是理解句子的含义。“我们必须保持屋子干净，要知道灰尘总是伴随疾病。(灰尘和疾病总是手拉手。)”故B项符合题意。
