For which items is multiple reference a concern in regard to the consistency of the objects value over time? (select multiple) bool int float tuple frozenset str list set dict Which of these is a valid way to create a 2D list of o's, assuming rows and cols are positive integers? (select multiple) a) arr: List[List[int] = [[0] * cols] rows = b) arr: List[List[int]] [] for i in range(rows): arr.append ( [0] * cols) c) arr: List[List[int]] = [[0] * cols for i in range (rows)] d) arr: List[List[int]] = [] for i in range (rows): arr.append([]) for j in range(cols): arr[i].append(0)