If 5a + 7b = g, where a and b are positive integers, what is the largest possible value of g for which exactly one pair of integers (a, b) makes the equation true?  Choose the correct choice from the following choices
A) 35	B) 48	C) 69	D) 70	E) 74
5*a1 + 7*b1 = g   5*a2 + 7*b2 = g   5*(a1 - a2) = 7*(b2 - b1)   since we are dealing with integers we can assume that a1 - a2 = 7*q and b2 - b1 = 5*q where q is integer, so whenever we get a pair for (a;b) we can find another one by simply adding 7 to a and subtracting 5 from b or vice versa, subtracting 7 from a and adding 5 to b.   Lets check how it works for our numbers, starting from the largest:   E)74 = 5*12 + 7*2 (a1 = 12, b1 = 2), subtract 7 from a and add 5 to b respectively, so a2 = 5 and b2 = 7, second pair - bad   70)70 = 5*7 + 7*5 (a1 = 7, b1 = 5), if we add 7 to a we will have to subtract 5 from b but b can't be 0, so - no pair, if we subtract 7 from a, we'll get a = 0 which also isn't allowed - no pair, thus this is the only pair for (a;b) that works,good!, thus   70 is the answer
<result>D</result>
If 5a + 7b = g, where a and b are positive integers, what is the largest possible value of g for which exactly one pair of integers (a, b) makes the equation true?