A firm's short-run production function (capital, K, is fixed; labour, L, is the variable input) has the reduced form:

Q(L)= 3L+12L²-L³

for L > 0.

Over what range of input does this function exhibit diminishing marginal product of labour?

1)Marginal product of labour is diminishing for all values of L < 4

2)Marginal product of labour is never diminishing for any values of L

3)Marginal product of labour is diminishing for all values of L > 4

4)Marginal product of labour is diminishing for all values of L