If two of the four expressions a+b, 3a+b, a-b, and a-3b are chosen at random, what is the probability that their product will be of the form of a^2 + kb^2, where k is a positive integer?
Choose one:  A) 1/2  B) 1/6  C) 1/4  D) 1/3  E) 1/2
*3=12 --&gt; total number of outcomes   but only two of the expressions will result in desired form --&gt;so the probability of choosing the right pair is 2/12=1/6   ANS:1/6
<result>B</result>
If two of the four expressions a+b, 3a+b, a-b, and a-3b are chosen at random, what is the probability that their product will be of the form of a^2 + kb^2, where k is a positive integer?