There are three basic types of p

There are three basic types of payment gateways. The first is 21_____ API (Application Programming Interface). 22_____ means that the customer never sees the payment gateway.
Web site —your shopping cart 23_____ to it seamlessly in the background. This is generally the best 24_____ as it's a transparent experience for the shopper, rather than being transferred to 25_____ site at the crucial moment of taking the money.
APIs 26_____ a bit scary, but your shopping cart vendor should 27_____ the hard work to support it, so there's very 28_____ work for you, the shop owner, to do. The only catch with this option is you'll need a secure certificate 29_____ on your server. These start at around $ 100 per year and work 30_____ way up quickly.
29.
A、instal
B、installing
C、installed
D、to install
【正确答案】：C