by, _____ go out to contact other people inperson, more and more people tend to socialize with others via electronicdevices

My pen friend and I keep in touch ________ sending e-mails often[ ]

The English-Chinese dictionary _____I bought many years ago is still of great value.

With few people ________ pigs, the price of pork _______

There’re more than 100 elements found in nature, ______ are metals.