题目

下列计算正确的为（■）.A．a2＋b2 = a2b2B．a2b－ab2=0 C．a2＋a2= a4D．3a3－2a3= a3 答案：D解析:略—Is Bob still performing?   —I'm afraid not. He is said _____ the stage already as he has become an official.     [     ]A. to have left               B. to leave    C. to have been left         D. to be left