I often have conversations with John over telephone, while keep in touch with Tom by letter.
A. 不填；the
B. 不填；a
C. the；不填
D. the; a
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(答案→)A. 不填；the
B. 不填；a
C. the；不填
D. the; a
解析：选 A。over [on] the telephone 为习语，意为“通过电话”；若用 by telephone 则不用冠词，类似地，by letter（通过信件）也不用冠词。