题目

饲养员养了白兔、黑兔共72只,白兔的数量是黑兔的3倍,白兔、黑兔各有多少只? 答案：解：设黑兔有x只,白兔有3x只。 x+3x=72x=183x=54 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