(2)根据课文内容填空(8分) There are two main types of advertisements— commercial advertisements and     41    service advertisements (PSAs). A commercial advertisement is one which someone has paid for to    42       a product or service. PSAs are often placed for free, and are    43     to educate people about health., safety, or any other issue which affects public    44       . This   45  will give you some information about RealCine: how it works, why it is better than   46   cinema, and how it can be used in other ways. I believe you will be     47   by the RealCine experience, and will agree that this is an extraordinary     48     that must be developed further.