7、中选出可以填入空白处的最佳答案.()21.Ihavetopracticethepianoforhoursaftersuppereveryday.It’sreally.A.playing,frustratingB.playing,frustratedC.play,frustratingD.toplay,frustrated()22.I find ___ useful to learn how to use a computer.A.that B.this C. its D.it()23.______isreallyhard____