问题：

[单选] When BGP tuning is used， how is packet flow into the e-commerce module controlled？（）

A . by tracking the status of objects along the path to the e-commerce module
B . by detecting undesirable conditions along the path to the e-commerce module
C . by using the MED to communicate the site preferences for traffic to multiple ISPs
D . by communicating the available prefixes， routing policies， and preferences of each site to its ISP
E . by moving the SLB to a position where selected traffic to and from the servers does not go through the SLB