Physics is difficult .A. to be learnedB. to learnC. to be learningD. to be learn

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A. to be learned
(答案→)B. to learn
C. to be learning
D. to be learn
解析：在easy, hard, difficult, light, heavy, interesting, dangerous, fortable, important, fit, pleasant, nice ,good等形容词后的不定式用主动形式表示被动意思。