Brain, like muscles, also needs some kind of exercise and nutrition. The more you use your brain in the right direction, the more you ________ enhance your ability.

—Do you know where Mary is? I couldn’t find her anywhere.—Well . She        have gone far—her coat’s still here.

I tried more than once to persuade the taxi driver to slow down, but he just __________ listen.

—Jenny，how did your math exam go?—I thought I ________，but in fact I came in the top 10% in the class.

_____ people in the world are sending are receiving e-mails every day.