Sentence: Humor is one of the best indicators...?

[Schwartz]

1. ____________ a tablespoonful of soil can tell so much about the structure and early history of the Moon.
a) It is remarkable that b) It is a remarkable fact that
What's the answer and why?

2. Humor is one of the best indicators ______________.
a) to show how does a generation think
b) of how a generation thinks
c) that tell about the thought of a generation
Why the correct answer is c) not a) or b), why i can't use them?

3. A number of physicians say that smoking is ________ as cholera and tuberculosis were to earlier generations.
a) as an important cause of death today b) as important a cause of death today
My answer is a) but it's the incorrect one and I don't know the reason why.

4. Manufacturing, principally of steel products, has in recent years become a leading industry.
Why does it use 'principally' instead of 'principal' ?

5. Bookkeeping is limited to dealing with business having financial aspects.
to + v.ing +prep.??? Why don't we use 'to deal with'?

[Mister Micawber]

1. What's the answer and why?-- Both are fine, although I suspect that the writer intended B to be the answer.

2. Why the correct answer is c) not a) or b), why i can't use them?-- The correct answer is B. Word order is wrong in A (it should be 'how...thinks') and C is redundant, awkward and should employ the plural 'thoughts'.

3. My answer is a) but it's the incorrect one and I don't know the reason why.-- B has the correct structure: as + adjective + a/an + noun + as.

4. Why does it use 'principally' instead of 'principal' ?-- 'Principally' (= mainly, for the most part) is an adverb not closely related to the adjective 'principal'.

5. Bookkeeping is limited to dealing with business having financial aspects.
to + v.ing +prep.??? Why don't we use 'to deal with'? 'Limit'. like some other idiomatic verbs, takes the -ing form: limit (someone) to doing; look forward to doing. 'To' is not the infinitive particle; it is a preposition.
_________________
Native English teacher at Mister Micawber's

[Schwartz]

Thank you for your explanation.

All of these sentences are from TOEFL. 1. correct answer is A but I don't really know why, I also think just like you 'both are fine'.
My answer for 2. is B but it's incorrect, and again I don't know why it is.

for 4. Manufacturing, principally of steel products, has in recent years become a leading industry.
Does it actually need adj.+of, doesn't it????

[Mister Micawber]

They are not from TOEFL itself; they are from a source that is writing practice questions like TOEFL's. The writer or book has made a mistake with 2. For correct grammar, 4 needs 'for'.
_________________
Native English teacher at Mister Micawber's

[Schwartz]

what do you mean ,4 needs 'for', ?

[Mister Micawber]

Sentence number 4 requires the word 'for' in formal grammar.
_________________
Native English teacher at Mister Micawber's

[Schwartz]

Where should I put 'for' in that sentence. Would you please give me the whole complete sentence.

[Mister Micawber]

Sorry, sorry! I meant 'of'-- the 'of' after 'principally'.
_________________
Native English teacher at Mister Micawber's