My friend and I always keep _______ touch by mail. When I get a letter, I usually write _______ immediately. A. off / upon B. on / about C. in / back D. by / to
My friend and I always keep _______ touch by mail. When I get a letter, I usually write _______ immediately.
A. off / upon
B. on / about
C. in / back
D. by / to