A security appliance that has been installed for six months and was designed to use artificial intelligence $($AI$)$ to detect malicious traffic on the network did not catch the existence of an APT that was discovered when it acted on its objectives, disabling the majority of the enterprise's critical systems. Analysis revealed that the original attack before establishing persistence occurred four and a half months ago. Why might this advanced Al appliance have missed such a threat?

A The attack was based on a technique too basic to be detected by such advanced equipment.

B The attack came from outside the network.

Tainted training data was used.

D The advanced nature of the threat allowed it to remain stealthy and bypass the intelligence of the appliance.