Fifty dollars （） a big sum of money for a child.

单项选择题The females in the rat population were （） by the high population density.

A．most seriously affected
B．the most seriously affected
C．the most serious affected
D．the serious most affectect

单项选择题It was not until almost a century （） blacks began to demand their rights as American citizens.

A．when
B．since
C．that
D．which