What value is stored in register $s0 after the MIPS code below executes?

Assume the values 5 and 4 are stored in registers $t0 and $s1, respectively.

add $s0, $s0, $s1 add $s0, $s0, $s1 add $16, $s1, $zero

Is it?

a. 5

b. 4

c. 9

d. 13