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

问题补充：

Physics is difficult .

A. to be learned

B. to learn

C. to be learning

D. to be learn

答案：

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, comfortable, 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。