A production function that depends on the square root of one input

Suppose that a firm has the production function f(x1, x2)=sqrtX1 +x(2,2)

a. The marginal product of factor 1 (increases, decreases or remains constant) as the amt of factor 1 increases. The marginal product of factor 2 (increases, decreases or remains constant) as the amt of factor 2 increases.

b. This production function does not satisfy the definition of rising returns to scale, constant returns to scale, or decreasing returns to scale. How can this be?

c.Choose a combination of inputs such that doubling the amt of both inputs will be more than double the amount of output. X1=4, X2 =4 for example

d. Choose a combination of inputs such that doubling the amt of both inputs will be less than double output. x1=2, x2=4, for example.