Some economists predict that the dollar will remain weak _______ economies abroad are expected to

grow faster than that of the United States.  [ ]

A. while

B. as

C. before

D. until

一般容积大、水深深的湖泊，通常水温的垂向分布有三个层次，上层、中间层(温跃层)和下层，其中溶解氧浓度最高的是( )。

A．上层
B．中间层
C．下层
D．三层溶解氧浓度均接近