题目

 ---- Why does the lake smell terrible? ----Because large quantities of water________ A. have polluted   B. is being polluted   C. has been polluted  D. have been polluted 答案：D9.5gMgCl2固体溶于水配成500mL溶液，其浓度为        mol·L-1，从中取出100mL溶液，其中Cl－的浓度为       mol·L-1；再把取出的溶液加水稀释成500mL，则此时MgCl2溶液的浓度为        mol·L-1。