Screen.

All other stuff tends to cover something viz., curtain covers windows, veil covers face, door covers room, shutter covers shop, etc. But screen can’t cover anything. 😛

Nothing. 😀

20 9s. 😛

979.

17-1 = 16 = Square of 4.
98-17 = 81 = Square of 9.
354-98 = 256 = Square of 16.

So, 4,9,16 becomes a new series with increments with digits in order of: 5, 7..
i.e., increments with +2. So next digit is 5,7,9,– (7+2 = 9)

Hence, add this 9 to 16 gives 25. So square of 25 is 625.

Add 625 to 354 which gives 979.

Series will be:  11,15,19,18,26,21,32, 24,37, 27, 41, 30, 44, —

There are 2 series going on: 11, 19, 26, 32, 37, 41, 44, —-
and 15, 18, 21, 24, 27, 30,—

First, series decreases by the difference of 2 consecutive and second increments by +3.