What is the security issue in classic packet filtering of active FTP sessions?

What is the security issue in classic packet filtering of active FTP sessions?

A.
The established keyword can’t be used for control or data sessions

B.
Allowing control sessions to the client opens up all the high ports on the client

C.
Allowing data sessions to the clientopens up all the high ports on the client

D.
The control session can’t be adequately filtered