st.1: It does not tell you the relative position of r and t. Not sufficient.
St.2. If s>0, then 0 is halfway between r and s. If s<0, r<0. So zero cannot be half way between r and S. Not sufficient.

Therefore, the answer is C.

Hi Tanya,
I would explain it as follows

stmt 1: As TT explained NOT SUFF

stmr 2: now if the distance of t from r is same as -s then we will have two cases

case 1: -s is to the left of t on the nos line then r=-s
This will arise when s is +ve

here we can say for e.g that s>0 so -s <0 & r= -s and both will be equidistant from 0
hence we can say that 0 is half way between r & s

case 2: if -s is to the right of t on the nos line then it is not the same point as r only having same distance from t as r.
This will arise when s is -ve

here we can take an e.g viz s<0 i.e -ve so -s will be +ve

now r is to the left of s on the nos line so r is also -ve

hence now 0 will not fall between them at all.

so INSUFF

Combine both then Case 2 will not arise as s is to the right of 0 i.e +ve hence only case 1 will exist, hence SUFF