构成毛细血管壁的细胞层数是（ ）
A：一层
B：二层
C：五层
D：十层
① ____ A. I like playing yoyo.
② ____ B. My father likes hamburgers.
③ ____ C. I like eating pizza.
关于电流的下列说法中正确的是（ ）
旅游已经成为现代人比较普遍的休闲方式之一。
____ A. I like playing yoyo.
____ B. My father likes hamburgers.
____ C. I like eating pizza.