几道英语题没想通，求讲解

In some places,little()to make smoke clean.
为什么一定是is done 不能是 has done
这是一个被动句，如果选第二个，应该是has been done

when the dog was only () old,he saved a drowning boy.
为什么是a year and a half 而不是 one and half a year
一年半的表达可以是one year and a half 和 one and a half years

There are () in our class
为什么是Wangs而不是 the Wangs?
the Wangs是姓王的一对夫妇，wangs是姓王的一些人

there're () fish sold at this market.
这里怎么会是a great number of? fish没+s来着……要不是我题抄错了？
是对的啊 fish是不可数，a great number of加可数名词的复数，而fish和people一样，单指鱼时单复同形

people often go to () church on Sundays in western countries.
为什么括号里不用填a?
go to church和go to school一样，前者表示去教堂做礼拜，后者表示上学，要是加了a就表示去一个教堂了

Do you know anything about Solomon,() king of ancient Israel?
为什么括号里不用填the
表示职位的名词前不加冠词，如Bush,president of USA

() Mrs.Williams () her husband has been to Thailand.Either of them can be your guide.
为什么一定要not only,but also 不能是 eith