题目

叶片与外界环境之间进行气体交换的“窗口”是（ ） A . 表皮 B . 叶肉 C . 叶脉 D . 气孔 答案：D— Are you going to travel during this summer vacation?— No, I’ll have to do some part-time job for my study. __________, there’s a disease spreading.A．AlsoB．ThereforeC．HoweverD．Unluckily