F. You can tell by ratio of side lengths, which is 1.5, which means that one side of triangle DEF is 1.5 times longer than the corresponding side in triangle ABC. We know that line AC corresponds to line DF because 9 is 1.5 times larger than 6. We also know that corresponding side lengths have corresponding angle measures, so we know that angle C corresponds to angle F.
C is just a way to say the order the books are picked in is random and ! means factorial. Factorial is when you multiply all the previous numbers up to the number it is. For example, 5! = 1 x 2 x 3 x 4 x 5 and 2! = 1 x 2
