Because the diamond is the hardest naturally substance , it is used in industry for to cut ,grinding ,and boring other hard materials .

我想问为什么用naturally 修饰substance呢？

The elbows are joints that connected people's up arms with their forearms .

我想问为什么不选C呢？很显然着说的是一个事实，答案选d，不过字典中up也用adj的用法。