takes a lot of concentration and a great sense of balance to stay on it.A. ItB. ThisC. Wh

问题补充：

takes a lot of concentration and a great sense of balance to stay on it.

A. It

B. This

C. What

D. That

答案：

(答案→)A. It

B. This

C. What

D. That

解析：句意：这需要高度的专注和很好的平衡感才能站在上面。这里使用it做形式主语，句型是:It takes sth to do …..，it是形式主语，不定式是真正的主语，所以选A。