求救三道语法题！！

我被困扰多时，急盼大家指点！！

97－10－33

The Hopi, the westernmost tribe of Pueblo Indians, have traditionally live large. multilevel structures clustered in towns.

97－12－30

30. The best American popular music balances a powerful emotions of youth with tenderness, grace, and wit.

97－12－16

Eleanor Roosevelt set the standard against which the wives of all United States Presidents since have evaluated.

第一题的tribe为什么对？

第二题虽然答案应该选B，但是为什么可数的youth单独存在？请问各位大虾什么情况下可数单被允许单独存在呢？除了part类的词？

第三题中，which是是against的宾语从句还是定语从句，该如何区分呢？这里的since是副词吗？谢谢各位！！！！！！急待回音。

我在后续的做题中，被类似这样的选项干扰多次，所以希望大家多多指教。

1、介词的宾从还是先行词的定从，如何区分？怎样理解？

2、可以单独存在的可数单有那些类别？

先谢谢了。

上来看了好几次，都没有大家的解答

谢谢！

第一题tribe是可数名词。由于谓语动词是have因此，要改成tribes

以下是朗文解释：

/ traɪb / [countable]
1
a social group consisting of people of the same race who have the same beliefs, customs, language etc, and usually live in one particular area ruled by their leader
a tribe of Aborigines known as the Dolphin People
2
a group of people with the same interests - used especially to show disapproval
tribes of journalists
3
a group of related animals or plants
the cat tribe
4
humorous a large family
We were only expecting Jack and his wife, but the whole tribe turned up.

第二题 youth 这里是不可数名词；见朗文解释

/ juːθ /

plural youths / juːðz $ juːðz, juːθs /
1
[uncountable] the period of time when someone is young, especially the period when someone is a teenager
ￚsee also old age
something of somebody's youth
the dreams of his youth
in somebody's youth
Many of these people had used drugs in their youth.
2
[countable] a teenage boy - used especially in newspapers to show disapproval
a gang of youths
3
[uncountable] young people in general
the youth of something
The youth of today are the pensioners of tomorrow.
4
[uncountable] the quality or state of being young
Despite his youth, he had travelled alone. The cream will restore youth and vitality to your skin.