We don’t write to each other regularly，but we do keep in
touch by telephoning .
A. at a time
B. once
C. at once
D. once in a while
网友回答
A. at a time
B. once
C. at once
(答案→)D. once in a while
解析：once in a while“间或，偶尔”;at a time 每次，一次；once 一旦；at once 立刻，马上。题意为：我们不经常相互写信但我们偶尔会通电话。