Just need a small explanation for each so I can understand the logic, thank you!

Problem 9: For each of the following, indicate whether True or False (you need not explain, but know your reasons rather than guess when answering). a) If it is proven that an algorithm takes O(n2) worst-case time, is it possible that it takes O(n) on some inputs? b) If it is proven that an algorithm takes O(n2) worst-case time, is it possible that it takes O(n) on all inputs? c) If it is proven that an algorithm takes (n2) worst-case time, is it possible that it takes O(n) on some inputs? d) If it is proven that an algorithm takes (n2) worst-case time, is it possible that it takes O(n) on all inputs