微观经济学的问题！求解释

A) if its hamburgers are perfect substitute for other fast food, then the other fast food will also be perfect substitution for its hamburgers. Increasing the price of hamburgers will force customers to switch to cheaper subsitutes, in this case "other fast food".
B) differentiated means there are no perfect subsituttion, i.e. not very elastic
C) perfectly elastic means any change in price will cause it to lose all customers
D) it means its subsitution decrease price, which will worse the situation
E) low barriers to entry implies lower cost of entry and exit meaning lower cost and eventually lower price of subsitution of hambergers in this case.

这应该是A-LEVEL难度的吧？不懂继续问