Words that are identical only in spelling but different in sound and meaning are called ______.

Thanks to the militant feminist movement of the suffragettes before the First World War, votes were granted to women over the age of ________ in 1918.

翻译：老年人有长处，但也有很大的弱点 —— 老年人容易固执，因此老年人也要有点自觉性。

Robots differ from automatic machines ________ after completion of one specific task，they can be reprogrammed by a computer to do another one.

Which of the following is NOT one of the main sources of new words in the present-day English vocabulary?