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 control session cannot be adequately filtered.

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

C.
The established keyword cannot be used for control or data sessions.

D.
Allowing data sessions to the client opens up all the high ports on the client.