题目

I often have conversations with John over _____telephone, while I keep in touch  with Tom by _____letter. A ./; the                  B. the; /              C. /; a                  D. the; a 答案：B将一木块按4：1的比例分成大小两木块，则大木块与小木块的密度之比是    ；甲乙两实心物体的质量比是2：1，密度比是1：2，则甲乙两物体的体积比是    ．