单项选择题
______ the ______ of modem industry, the pollution is becoming more and more serious.
单项选择题 If you don't believe me, you'd better go to have a look at it ______ person.
单项选择题 With the guide leading the way, the visitors were able to travel through the forest not by air but ______ foot.
单项选择题 Two dogs ______ a bone, and a third runs away with it.
