下列叙述中，正确的是（    ）

A．二氧化碳分子是由一个碳原子和一个氧分子构成的

B．不同元素的原子其核电荷数一定不同

C．原子是构成物质的最小微粒

D．核外电子数相同的微粒一定属于同种元素

Even after having their grandchildren live with them for ten years, the couple felt that ________children these days was the most difficult of all family matters.

A．rising

B．raising

C．caring

D．taking care