题目

 She went out, ________ her baby by itself.          A. leaving               B. left                 C. having left             D. to leave 答案：  A18．已知函数f（x）=cos（asinx）-sin（bcosx）无零点，则a2+b2的取值范围为[0，$\frac{{π}^{2}}{4}$）．