By find how high the water would be after 20 seconds, you would find out much height is increasing each second.
21 - 4 = 7
28 - 21 = 7
35 - 35 = 7, and so forth.
So, we know that 7 cm is increasing each second. You would just add 7 to the height for each second.
2 --> 14
3 --> 21
4 --> 28
5 --> 35
6 --> 42
7 --> 49
8 --> 56
9 --> 63
10 --> 70
11 --> 77
12 --> 84
13 --> 91
14 --> 98
15 --> 105
16 --> 112
17 --> 119
18 --> 126
19 --> 133
20 --> 140
Thus, after 20 seconds, the water's height would be c) 140 cm.
