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1.Humus,a substance(A) found in soil,is soft and spongy and(B) enables plant roots to send out tiny hairs(C) through that(D) they absorb water and food. I dont understand why D IS the answer.(choose the wrong word)
2.today____little fossil ivory remains comes from Alaska. A.what
B.WHICH C.THERE IS D.WHERE The answer is A But i choose C,so why choose A?thank you
3.In jazz music, a riff is a simple melodic figure, _______ and repeated several times.
(A) usually one or two measures lasted
(B) lasted one measure or two usually
(C) is lasting one or two measures usually
(D) usually lasting one or two measures
which is the answer
4. Living organisms contain more water _______ substance.
(A) than do any other
(B) does than any other
(C) other than do they any
(D) than they do any other

which is the answer?
Comments  
I can't be too technical but I will give it a go as a native speaker, and hope you find it helpful.

1) The answer is 'which'. through which they absorb water not through that. You might say 'that they absorb water through' but you have to say 'through which they absorb water'. Explaining exactly why is beyond me though.

2) The correct answer is what. The sentence means means that there is only a little fossil ivory in the world, and what there is, comes from Alaska. Remains is being used in the sense of what is left, not as in fossils or skeletons! You might understand it better if you punctuate it: Today what little fossil ivory remains, comes from alaska.

3) The correct answer is D. The other answers are not in the same tense.

4) the correct answer is also D.
A would be saying that living organisms have more water than even ...a glass of water? No.
B and C just do not make any sense.
1.) This question is a little hard for me to follow, but that sentence should read this way, "Humus, a substance found in soil, is soft and spongy and enables plant roots to send out tiny hairs t hrough which they absorb water and food." That's how it sounds best.

2.) This answer is a little difficult to explain. The reason A is correct and C isn't...
Well, "Today, there is little fossil ivory remains comes from Alaska" just doesn't make sense. The reason you use "what" in that sentence...hmmm...well, if you changed it into a question, it would be, "what little fossil remains?" English is weird in that some of the grammar rules revolve around totally changing the meaning of a sentence. I'm having a slow day with my brain so maybe somebody else can explain this better? Sorry.

3.) D is the answer.

4.) D is also the answer here.

Hope this helps some.
Students: We have free audio pronunciation exercises.
thanks,you two guys,but the first question still puzzles me,why not use"that"?