在下面的商品组合中，哪些是互补品，哪些是替代品？在不同的情形下，它们可以两者兼而有之吗？请论

(1)一堂数学课和一堂经济学课;

(2)网球和网球拍;

(3)牛排和龙虾;

(4)目的地相同的飞机旅行和火车旅行;

(5)熏肉和鸡蛋。

Which of the following combinations of goods are complements and which are substitutes？ Could they be either in different circumstances？ Discuss.

a. a mathematics class and an economics class

b. tennis balls and a tennis racket

c. : steak and lobster

d. a plane trip and a train trip to the same destination

e. ; bacon and eggs