第28题: [单项选择]It didn't seem to have ______ to her that once she was addicted to drug smoking, she could hardly give it up. But it did happen. A. happened B. took place C. occurred D. came about 参考答案:C 答案解析:happen to sb.…(事件)临到……头上,发生在某人身上;选项B时态不正确,排除; occur to sb.(主意)浮现在某人的脑海中,被某人想到;选项D时态不正确,排除。此题的意思是她从没想
第30题: [单项选择]n元线性方程组Ax=b有唯一解的充要条件是( ) A. A为可逆的方阵 B. 齐次线性方程组AX=0只有零解 C. A的行向量组线性无关 D. 矩阵A的列向量线性无关,且向量b可由A的列向量组线性表示- 参考答案:D 答案解析:矩阵A可逆是方程组AX=b有唯一解的充分不必要条件,(A)不对; 若AX=0只有零解,则r(A)=n,但不能由此推出[*],(B)不对; A的行向量组线性无关只能保证A行满秩,