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

I. Constraints with the form "Amount of resource used $$ Amount of resource available"

II$.$ Constraints with the form "Amount of resource used $$ Amount of resource available"

III. Constraints with the form "Level achieved $$ Minimum acceptable level"

IV$.$ All constraints have the form "Amount provided $=$ Required amount"

Multiple Choice

I only

II only

III only

IV only