请教0505TOEFL几个语法题

以下四题很是不解 请大家赐教 不胜感激！

改错：

28. Methane gas in the upper atmosphere account for the planet Uranus' greenish hue. (是应把the去掉，还是应在account后加s？不可名词作主语 动词是否一定要要s?)

32. In medical and dentistry, radiography is invaluable for diagnosing bone damage tooth decay, and internal disease. (medial错了？ 还是应把tooth改为teeth？）

填空：

15. Four flags have flown over the Palace: ___ Spain, Mexico, the Confederacy, and the US. (which of/those of/those that of) 我觉得应该选 those of 吧 但不确定

8. The ___ east of the Mississippi River is ....     ( largest Native American nation / largest nation Native American) 哪个正确呢？

个人意见。请高手指正。

28. accounts for

32. medical

15. those of

8.  largest Native American nation

同意楼上的