--- Is this the first time you ___ Xiamen? --- No. But the first time I ____ here, the city wasn’t so beautiful.

Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.

 We drew this conclusion on the _______of research and experiments.

When he realized the audience’s attention was beginning to ___, he raised his voice. 

 What I’m interested in is radios and telephones, so I’d like to study __________ Engineering during my four years in university.