We must---practising speaking English as much as possible.

答案：选C.We must 【keep on】practising speaking English as much as possible.
解析：这道题目主要是考察你对keep up with,catch up with,go on和keep on这四个动词词组的使用.keep up with,catch up with都是追上,赶上的意思,在这句话中与句意不符；而go on和keep on都是继续做某事的意思,但是B选项中是go on with,这个with是多余的,所以B选项也是错的,只有C选项符合句意,且语法正确.
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