前运算阶段的思维特征是（）。
A.单维思维
B.思维的不可逆性
C.具体逻辑推理
D.思维的灵活性
E.自我中心
参考答案：A, B, E
选择正确答案。
( ) 1. Li Ming are getting ready (a. for b. to) school.
( ) 2. Mitts and mittens are (a. different b. the same).
( ) 3. Look (a. out b. on) of the window.
( ) 4. I like to play (a. on b. in) the snow. Let's go (a. inside b. outside).
( ) 5. I have (a. a b. two) sticks (a. for b. at) the snowman's arms.
分子标记的优点是什么？
