题目

Joan has been missing for a couple of days and there is growing _____for her safety.    A.attention    B.danger     C.concern     D.hope 答案：C3．将下列方程化为有理方程：|$\sqrt{（x+c）^{2}+{y}^{2}}$-$\sqrt{（x-c）^{2}+{y}^{2}}$|=2a（其中0＜a＜c）．