A Text completion that seems to me very difficult. Could anyone explain this.....

Though dealers insist that professional art dealers can make money in the art market, even an ________ knowledge is not enough: the art world is so fickle that stock-market prices are ________ by comparison.

Blank (i) Blank (ii)
A amateur’s D booming
B investor’s E erratic
C insider’s F predictable

What do you think, Shakhaowat?

Start with the second word as only one of the possibilities could work there. A clue is that the market is described as ‘fickle’.

If the art world is quite fickle/changeable then the stock-market prices will also be erratic if we compare. Am I right?

That’s right - so it should be easy to work out the rest now.