Vapour pressure of solution

[Ice-cream]

can any1 help me with this?

 

"Which of the following will have the lowest total vapour pressure at 25 degrees C?

a) pure water (vapour pressure = 23.8 torr at 25 degrees C)

b) a solution of glucose in water with X(C6H12O6) = 0.01)

c) a solution of sodium chloride in water with X(NaCl) = 0.01)

d) a solution of methanol in water with X(CH3OH) = 0.2 (Consider the vapour pressure of both methanol [143 torr at 25 degrees C] and water)"

 

I'm not sure whether it would be right if, say for b), I go P(total) = 0.99 x 23.8 = 23.562 (coz P of water would be 1 - 0.01 = 23.8)...i don't know becuase if that's right, then I will get the same answer for both b) and c)...but the answer at the back of the textbook says solution C will have the lowest total vapour pressure.

so can any1 tell me what's going on?!?!