You did a pretty good job when ruling out A, but not for stmt 2.
For a sum of two numbers to be positive, you have three options:
a. z is positive and f is negative. Example: z is 6 and f is -1. z + f = 5 >0.
b. z is negative and f is positive. Example: z is -3 and f is 5. z + f = 2 > 0.
c. both numbers are positive, which means that their sum will always be positive.
Now, you can NEVER add two negatives and have a positive sum, since both of them are negative!
So, by using stmt 2, you get nowhere.

But when you combine the two, you get that z and f are both positive, since it is the only common case for the two statements. So C is correct.

Many Thanks.

Yes you are right. I did not evaluate statement 2 properly.

Thanks a lot for your help.