We may find that learning a foreign language does not have to be________learning our mother tongue,but can in fact be twice ________.
She would lend me the money if I promised to ________ within a month.
Water, _____ we cannot live, exists in the form of liquid, gas or solid.
His laziness ___________ his failing in the exam.
As we all know, smoking ___________ people’s health.
_____ you have worked out the first problem, you may go on to work on the second.