Correct answer with proper explanation

Jack could manufacture COVID 19 test kits using two processes. For process 1, the unit cost of the first two million kits is $10, and the unit cost is reduced to $4 afterward. For process 2, the unit cost of the first five million kits is $8, and the unit cost is reduced to $5 after that. Jack has a budget of 20 million dollars and likes to maximize the number of test kits. Formulate this problem as an IP. I suggest you write out the complete formulation before you answer the multiple-choice questions.

Decision Variable:

X11: millions of kits manufactured at $10 using process 1

X21: millions of kits manufactured at $4 using process 1

X12: millions of kits manufactured at $8 using process 2

X22: millions of kits manufactured at $5 using process 2

What is the objective function?