—When can I use your computer?—Never! ________should you touch it. 

It is generally believed that constant reading and comprehending can ______ our mental ability of comprehension while physical exercise can help build up our body. 

It is a good plan in theory, but it remains _______ whether it works in practice.

Some of my friends eat with their eyes. They prefer to order what   ______ nice.

Having lived in China for 8 years, Helen has a good ____________ of both English and Chinese.