In those days we had no phones so we have to keep in touch writing often.A. withB. ofC.

问题补充：

In those days, we had no phones, so we have to keep in touch writing often.

A. with

B. of

C. on

D. by

答案：

A. with

B. of

C. on

(答案→)D. by

解析：考查介词的用法。句意：在那些日子，我们没有电话，因此我们不得不经常靠写信保持联系。介词by指凭借某种方式，所以选D。