1. ) D
x-y >10
now if x=8
8-y> 10 ie y < -2
8+(-2) <8>10 ie x > -10
-10 + (-20) < -10 -(-20)
now if you make x as -5, 0, 5, 10... same result


2.) E - Not very sure
qc pc + qd pd = 6
qc,qd - quantity of c and d
pc,pd - quantity of c and d

3pc - 2pd = .10 ------------ 1
(pc +pd )/2 = .25
pc + pd = .7 -----------2

solve 1 and 2 you get pc and pd..... but u still need qc if you want value
of qd....

3) c
This is simple .. 1 point is given (0,4) ie y intersect is 4...
slope = y2 -y1 / x2 - x1

for (6,0) ... where x intersect is 6
0-4 / 6-0 = -4/6 which is -ve ..

for all other values it is +ve or cannot be found for 0