造成青少年近视的原因是

A．晶状体变形，发生实质性的病变

B．眼球的前后径过长，物象落在视网膜的前方

C．晶状体混浊，不能接受光的折射

D．眼球的前后径过短，物象落在视网膜的后方

An activity has an early start date-of the lOth and a late start date of the 19th. The activity also has a duration of 4 days. There are no nonworkdays. From the information given, （）what can be concluded about the activity?

A.Total float for the activity is 9 days.

B.The early finish date ofthe activity is the end ofthe day on the 14th.

C.The late finish date is the 25th.

D.The activity can be completed in 2 days ifthe resources devoted to it are doubled