The water diet requires one to drink two cups of water every half hour from when one gets up until one goes to bed, but otherwise allows one to eat whatever one likes. Four adult volunteers agree to test the diet. They are weighed prior to beginning the diet and after six weeks on the diet. The weights (in pounds) are

Person 1 2 3 4 Mean SD
Weight before diet 180 125 240 150 173.75 49.56
Weight after six weeks 170 130 215 152 166.75 36.09
Difference 10 -5 25 -2 7 13.64

The goal is to test whether the water 'diet' leads to a mean weight loss.

What is the null hypothesis? Let subscript 1 indicate the measurement before the diet, and subscript 2 the measurement after six weeks.

(Pick letter corresponding to the best answer.)

a - H0: x?1-x?2 = 7
b - H0: ?1-?2 = 7
c - H0: x?1-x?2 = 0
d - H0: ?1-?2 = 0