根据首字母提示填写单词所缺部分，使句子完整。
There’s nothing in the bottle. It’s e____________.
Please k_________at the door before coming into the room.
At the beginning of 18^th c_________, the population of the world was very small.
I’m very h_______. Please give me something to eat.
Mary looks unhappy. Let's have a try to c_________her up.