The key identifying feature of a mixed problem is which of the following?

Constraints with the form $$Amount of resource used $>=$ Amount of resource available$$

Constraints with the form $$Amount of resource used $<=$ Amount of resource available$$

Constraints with the form $$Level achieved $>=$ Minimum acceptable level$$

All constraints have the form $$Amount provided $=$ Required amount$$

Multiple Choice

I only

II only

III only

IV only

At least two of II$,$ III, and IV$.$