The newspaper says that six _____ people travel to work by train every day.
根据设空处前的 six 可知，符合题意的词是 million，故答案为选项B。
There weren't many tall buildings in Shenzhen in _____.
表示“在二十世纪七十年代”要用 in the 1970s，故答案为选项A。
You'll get better grades _____ you follow the teachers' advice.
句意：如果你听老师们的建议，你将取得更好的成绩。根据句意可知答案为选项A。
— _____ is the population of Cambridge?
— It's about 120,000.
询问人口数量可以使用 "What's the population of…"的句型，故答案为选项C。