Mom often has conversations with my sister over ________ phone, while keep in touch with me by ________ letterMom often has conversations with my sister over ________ phone, while keep in touch with me by ________ letter.
A.不填; theB.不填; aC.the; 不填D.the; a在线课程C
解析：
第一个空填the是特指那部电话；
第二个空不填；
by letter固定搭配，用通信的方式第一个空填the是特指那部电话；
第二个空不填；
by letter固定搭配，用通信的方式。