we are experimenting a bit with the time the curtain goes up

一道语法题
SAT的 改错
Researchers are experimenting with various techniques for preventing the accumulation in water of high levels of nitrogen, which can kill plants and animals.
为什么这句话是对的, which 指代的是nitrogen啊.怎么能说nitrogen kill plants and animals

一个英语问题
Scientists have been experimenting with this torpedo-shaped glider since2007.为什么要用have been experimenting ,现在完成进行时?

请教大神们一道SAT语法题!
Researchers are experimenting with various techniques for preventing the accumulation in water of high levels of nitrogen,which can kill plants and animals.
which修饰的是什么,以及怎么在其他题目中判断which修饰的是什么,THX!

英语翻译
on August 12 1887,edison began experimenting with something which he had made himself!

英语翻译
It definitely prepared us,experimenting and trying different things and seeing what worked for us and what didn't.
night in and night out 该怎么翻译?

新概念英语第二册第二十二课
They began by experimenting ( ) rats,选填介词in/on/of/form其中之一.
帮忙选出正确答案并分析一下句子,谢谢!

用介此副词填空
We began experimenting______rats
The West believe______God
Please concentrate_______what you are doing
Many people escaped ______prison during the last five years
What does this horrible drink consist_________.
一会上课了

英语翻译
I've been experimenting with drugstore foundations for a couple of months.Generally there is *something* wrong with them.However,I feel the same about most of the high end ones as well.So far I am very pleased with this.Most importantly,the buff beige color is a terrific match for my difficult-to-match light yellow tending to redden complexion.Nearly everything out there is too pink/orange for me,of if pale neutral enough it looks chalky.Buff beige looks too dark in the tube,but lightens on the skin to a near-perfect match for me (worth it's weight in gold).The texture,too,is very nice.Light and silky,it blends into skin very quickly and relatively seamlessly.The finish is a touch too matte for my tastes,but does help with the oiliness of my t-zone.Really like the spf and nutrients added as well.
On the controversial "eraser" applicator front,I actually think it's a decent idea.Ideally you should use it to buff the product into the skin bit by bit,which is very nice if you can get it to do that as the sponge tip is very soft and goes around nose and under eyes like no other tool I've used.However,the product tends to flow out unevenly,so you must use your fingers to blend it,which is kind of a drag.Good idea but needs work,or just package the stuff in a nice glass pump bottle.