Why the answer is not irrational in the second blank?

irrational is the least favourable choice in this question.

even a(n) (i)------ knowledge is not enough suggests that something positive would go in for the first blank.

The art market is so fickle that the stock market would like predictable or stable in comparison.

The second blank also has to be something positive to say about the stock market.
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the art world is so fickle that stock-market prices are (ii) ------- by comparison; if the art world is changing, how something related is predictable??

The comparison is made to show the contrast between the two fields. For instance, if you say calculus is so complex such that it can make arithmetic look super easy. Similarly, you are saying that art is so fickle that it can make the stock market look predictable.

Makes sense now?
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Yes!!
Thank You

Though dealers insist that professional art dealers can make money in the art market, even a(n) (i)------ knowledge is not enough: the art world is so fickle that stock-market prices are (ii) ------- by comparison.


start with the second half of the sentence after the colon, we can make a clear comparison between the art world which is fickle and ever changing and the stock market which is static and non changing comparatively. so for the second blank erratic ( off course/ not fixed) doesnt work , irrational ( not justified) doesnt work hence the correct option for 2nd blank is predictable.

now moving to the first blank,

from the 2nd half of the sentence we know that the art world is fickle and difficult to pin down , hence it required extreme skill and dexterity to predict the movements of the art market that even an astute knowledge can ensure success. the best fit for the first blank is insider ( which means you have inside/recondite information about the trade). amateur and investor are wrong choices for the first blank.

can use some typical wordings..

Though dealers insist that professional art dealers can make money in the art market, even a(n) (i)------ knowledge is not enough: the art world is so fickle that stock-market prices are (ii) ------- by comparison.

Lets begin with this sentence fragment below after the colon.

the art world is so fickle that stock-market prices are (ii) ------- by comparison.

Comparison is being made between art world and stock market prices. And the art world is described as so fickle, that the only way to complete the sentence is to state that the stock-market prices are predictable by comparison.

Now lets go to the fragment containing the blank and before the colon.

even a(n) (i)------ knowledge is not enough

Now, we must keep in mind that what comes after the colon elaborates or defines what came before the colon. So the word for the blank should be such that it supports the idea that the art world is very fickle.

While we could say than an investors knowledge is not enough, the presence of "even" adds a level of emphasis, which makes us want to choose the extreme case - namely - even an insiders knowledge is not enough.

It is important to understand that the "though" at the beginning of the sentence (in the fragment though....market) is irrelevant to the solution and is nothing but a trap that we must avoid.

The correct answers are predictable and insiders.
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