Multiple Choice Questions Related To Testing Knowledge about Time and Space Complexity Of A Program

Why is 6th one d??

6th one is d because the outer loop is running n times and inner nested loop is running m times so O(nm)
Think of it like this inner loop is running m times for each i so m+m+m+…(n times) which is m
n or as written n*m

1 B
2 C
3 B
4 B
5 C
6 D
7 C
8 C
9 C
10 B
11 A
12 C
13 C
14 B
15 B
16 B
17 B
18 D
19 B
20 B
21 C

How to do Q 13 and Q 14 ?? Anyone Please help me to understand