Each context Permutation or Combination?

Context A: Jenny needs to determine the number of three letter arrangements of the letters of the word MATH.

Context B: Ross needs to find the number of 4-people committees that can be formed from a class of 20 people.

Context C: Rob needs to determine the number or arrangements of four books as part of a display shelf.

Context D: Deanna needs to determine the number of three digit security codes that can be formed from he digits 0-9 where digits cannot be repeated.