4. Express the answers to the following problems in scientific notation. Hint: Use the subtraction example to solve the addition problem. A. (1.8 X 1020) X (2.5 X 10^16) C. (9.2 X 10^19) ÷ (3.0 X 10^7) B.(7.9 X 10^9) - (6.0 X 10^8) D. (4.2 X 10^9) + (8.0 X 10^18)​