?忙于做某事

...B解析分析：少量的“be+形容词”后面的动词要用doing形式，常见的有be busy doing sth(忙于做某事)，be worth doing(值得做)。(延伸)be busy with sth(忙于做某事)和be busy doing意思相同。

?忙于

... get off离开; 动身 be busy doing sth忙于(做)... pick up拾起, 捡起; 抬起 ...

?忙于干某事

做完某事 be busy doing sth. 忙于干某事 go on doing sth. 继续干某事 .

?忙于做

... be busy with 忙于... be busy doing sth 忙于做... by bus /car / plane 乘公共汽车/汽车/飞机 ...