在利用光具座进行凸透镜成像的实验中，如图所示，一束平行于凸透镜主光轴的光线通过凸透镜后，在光屏上形成了一个最小、最亮的光斑．该凸透镜的焦距是_________cm．

She wondered __________.

A．how much he cost the computer

B．how much he paid for the computer

C．how much the computer will cost him

D．how much did he spend on the computer