please help me with the question. it may have more than one answers

1. y= F(1, 2) units of a certain product are produced using two inputs at levels c1 and x2 respectively. If w1, w2 and p are the input and output prices, the optimal choice of input demand for input i must necessarily rise if a. F is differentiable. b. Profit displays increasing differences with respect to (p, I;) for each i = 1, 2. c. F is concave. d. F displays increasing differences in the two inputs