A ratio could increase if:

1. the numerator increases and denominator remains constant
2. the denominator decreases and the numerator remains constant
3. the num increases and the den decreases simultaneously.
4. both num and den increase

St 1 just says the ratio increased is s/(s+b) increased. We don't know which of the above mentioned situations took place. INSUFF

St 2 is quite clear to you.

Combining the 2 statements.

We know den increased. So even the num has to increase for the overall ratio to increase. (4th situation).

Hence the answer is C.