许多录像带零售店提供两种可选择的租片方式:(1)两部收费法。支付一笔年度会员费(例如40美元)，然

许多录像带零售店提供两种可选择的租片方式:

(1)两部收费法。支付一笔年度会员费(例如40美元)，然后在日常租赁影片时付较低的费用(例如每片每天2美元)。

(2)直接付租赁费。不付会员费，但付较高的日租费(例如每片每天4美元)。

本例中两部收费背后的理由是什么？为什么要给顾客两种方案选择，而不是只有一种两部收费？

Many retail video stores offer two alternative plans for renting films:

a A two-part tariff: Pay an annual membership fee (e.g S40), and then pay a small fee for the daily rental of each film (e. g. $2 per film per day).

b. A straight rental fee; Pay no membership fee, but pay a higher daily rental fee (e.g $4 per film per day). What is the logic behind the two-part tariff in this case？ Why offer the customer a choice of two plans, rather than simply a two-part tariff？