一份由中国民间控烟机构发布的报告指出，目前我国青少年吸烟率为11．5%，约有1400万青少年烟民，青少年吸烟率呈现低龄化及上升趋势。吸烟对青少年而言是一种（   ） 

在晴天中午，密闭的玻璃温室中栽培的玉米，即使温度及水分条件适宜，光合速率仍然较低，其主要原因是（）

    Bob and Sue are in the same school，1they are in different classes.After school，Bob and Sue often play games with their friends.Class begins at eight in the morning.Now Bob and Sue are in2own classrooms.They are3to their teachers.Bob is studying English.Sue is having an art lesson.She4to make a birthday card5her mother.

    Her teacher Miss Li is helping her.

如图，AC是▱ABCD的一条对角线，过AC中点O的直线分别交AD，BC于点E，F．