How can a developer write this page to display the result of the web service and ensure governor limits for concurrent usage are not exceeded?

This sales team needs a custom Visualforce page to enter sales orders. When a product is selected on the Visualforce page, a web service is invoked to determine if the product is in stock, and the result is displayed on the page.

How can a developer write this page to display the result of the web service and ensure governor limits for concurrent usage are not exceeded?
A . Use Continuation that is invoked when a Submit button is clicked.
B . Use visualforce Remoting to handle the web service callout.
C . Use the Salesforce Metadata API in the web service callout.
D . Use an Apex trigger with callout=true annotation.

Answer: A