It’s important the piano well. A. of him to playB. for him to playC. of him playingD. for

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A. of him to play
(答案→)B. for him to play
C. of him playing
D. for him playing
解析：It is + adj.+ for/of sb.+ to do sth是固定句型.
It is+adj.+of sb. +to do sth中的adj.跟sb.有关,这个adj.是用来形容sb.的,表示某人这么做真是太adj.了。
It is +adj.+for sb.+to do sth表示做某事对某人来说很adj.，这里的adj.是do sth.的属性，里面的sb.跟adj.没有直接联系。
该句是说钢琴弹得好重要，而不是他这个人重要，故介词用for而不用of.