题目

_______ my sister three times today but her line was always busy. A. I’d phoned B. I’ve been phoning C. I’ve phoned D. I was phoning 答案:C 解析: 全句意思为“今天我给姐姐打了三次电话,但是她的电话总是占线”。首先依据时间状语today,排除A、D两项;现在完成进行时多表示持续不断的动作,用在此处不妥。故选C项。用纯种的黄色圆粒豌豆和绿色皱粒豌豆杂交,F1全部是黄色圆粒,F1自交得F2,在F2中杂合的绿色圆粒有4000个,推测纯合的黄色皱粒有(    ) A.2000个           B.4000个         C.6000个           D.8000个
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