why is the analysis often for the worst case? (select all correct answers) a. worst-case analysis shows the exact execution time b. best-case is not representative. c. worst-case is not representative, but worst-case analysis is very useful because you can show that the algorithm will never be slower than the worst-case. d. average-case analysis is ideal, but difficult to perform, because it is hard to determine the relative probabilities and distributions of various input instances for many problems.