题目

Fishing is a popular activity and every fisherman knows the rule: Keep the big ones, throw the smaller ones back. The idea behind it is simple —the larger fish are probably older. If you keep the smaller ones, they won’t be able to reproduce, and the fish population is in danger.But fishing out the largest fish from a population has an unwanted effect: Over time, fewer adult fish get really big. If only the smaller fish reproduce（繁殖）, then future generations become smaller. This is an example of evolution（进化）in action.One scientist, Dr. David Conover, has spent the last dec木块A、B分别重50N和60N，它们与水平地面之间的动摩擦因数均为0.25，夹在A、B这间的轻弹簧被压缩了2cm，弹簧的劲度系数为400N/m。系统置于水平地面上静止不动。现用F=1N的水平拉力作用在木块B上，如图所示，力F作用后 [     ]A．木块A所受摩擦力大小是12.5N B．木块A所受摩擦力大小是11.5N C．木块B所受摩擦力大小是9N D．木块B所受摩擦力大小是7N