D. If you are working hard, then you will get good grades.

You can use this equation for any numbers or variables in the form a2 - b2.
Area of a rectangle formula: A=LW
substitute the values in:
122.12=L(8.7)
Solve for L. Divide by 8.7 to isolate L.
L≈14.04 feet
'D' is correct as it is the only answer which includes commas when listing the characters and before "all of whom"
Try saying the sentence out loud and hearing where a pause is necessary.
Hope this helped!