单项选择题
Your father likes to play golf; he's really enthusiastic ________ it.
单项选择题 The tour of our country by the foreign visitors ________ on the workings of democracy.
单项选择题 Without the friction between their feet and the ground, people would ________ be able to walk.
单项选择题 Why does the author mention "planktonburgers" in (L. 3, Par
