Maybe he will come,but I'm not sure _____ that.
考查介词辨析。句意：也许他将会来，但是我不确定。此处的短语为 "be sure about"，意思是 “确信；对……有把握”。
Mr. Black,you are becoming healthier and healthier. Please keep on _____.
考查keep on的用法。句意：布莱克先生，你正变得越来越健康。请坚持跑步。根据keep on doing sth. 可知答案为running。
Amy practices(play) the piano for over two hours every day.
考查practice的用法。句意：埃米每天练习弹钢琴两个多小时。表示 “练习做某事”，应用practice doing sth.。
Be sure to let Tom know the notice as soon as he _____.
动词时态。
考查动词时态。句意：确保汤姆一到达就让他知道这个通知。在含有 as soon as 引导的时间状语从句的复合句中，主句是祈使句，从句要用一般现在时。故选选项C。