A cluster administrator decides ti implement application monitoring in order to monitor both a web application and its supporting database. The plan is to use a single resource group prod_rg and to configure one monitor method for the web app and one monitor method for the database. Which of the following statements best describes the need for custom resource monitor？（）

A. Multiple application servers must be in a single resource group.
B. Multiple application servers per resource group may be monitored.
C. Multiple resource groups must each contain a single application server.
D. Multiple application servers in a cluster cannot use process monitoring.