When to use "that" and when not to

[Abecedarian]

This is based on a question from

"Test No. incompl/elem-2 "Money accepted", answer 1"

I like to visit other countries but I find the cost of travel is too high.

Correct answer: (b) cost
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My doubt is:

a) I like to visit other countries but I find the cost of travel is too high.

Can the above sentence be replaced by the following?

b) I like to visit other countries but I find that the cost of travel is too high.

or

c) I like to visit other countries but I find the cost of travel too high. (no "is")

Are all three versions correct? If not which of these are wrong and why? If they are all grammatically correct, which of them is preferrable and why?

Thanks

[Torsten]

In sentence b) you simply add the conjunction that making the sentence a little shorter. In sentence c) you leave out the verb is converting the cost of travel into an object. All three sentences are grammatically correct.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Abecedarian]