I went to the library and read anything I could find about the mysterious disappearance of Bermuda Triangle ships and planes.
If you continue to treat your friends like this, you will lose all your friends. (and)
Keep treating your friends like this, and you will lose all your friends.
They must fully understand their products and know how to promote them by oral, telephone or letter. (by oral means)
They must know their products thoroughly and know how to promote sales by word of mouth, telephone or letter.
I suddenly realized that money can't make up for it.
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