$-$ Increasing the upper limits $($or reducing the lower limits$)$ to expand the range of feasible solutions is referred to as

a$.$ Restricting.

b$.$ Imposing.

c$.$ Loosening.

d$.$ Appropriating.

Alternate optimal solutions to LP problems exist when

a$.$ The feasible region for the problem is an empty set.

b$.$ One or more constraint in the problem formulation is redundant.

c$.$ The objective function can be made infinitely large or small, depending on the sense of optimization.

d$.$ There can be more than one feasible point that maximizes $($or minimizes$)$ the value of the objective function.