18. A certain city has two types of people: the 'positives', who only ask questions for which the correct answer is "yes" and the 'negatives' who only ask questions for which the correct answer is "no". When Mo and Bo met Jo, Mo asked, "Are Bo and I both negative?" What can be deduced about Mo and Bo?
A Both positive
B Both negative
C Mo negative, Bo positive
D Mo positive, Bo negative
E impossible to determine
SOLUTION
C

If Mo is positive, then the answer to his question must be "Yes" and that means he is negative, a contradiction. Hence Mo is negative. Therefore the answer to his question must be "No". So Mo and Bo cannot both be negative, and therefore Bo must be positive.