It is raining outside. Why not _____ an umbrella?
why not 后若跟动词，要用动词原形，故答案为选项B。
— There is someone knocking at the door.
— It must be the repairman. I _____ him to come to fix our fridge two hours ago.
本题考查时态
根据 two hours ago 可知，时态应用一般过去时，故答案为选项C。
To keep children safe,we _____ put the things like knives and medicine away in our house.
本题考查情态动词的用法
句意：为了保证孩子们的安全，我们应该把家里的刀和药之类的物品收起来。表示“应该”要用 should，故答案为选项B。
We can _____ some information about this city on the Internet.
本题考査 look 短语的含义
look up 查阅，査找，向上看；look like 看起来像；look after 照顾；look forward to 期待。句意：我们可以在互联网上査阅关于这个城市的一些资料。表示“査阅”要用 look up，故答案为选项A。