Confused on 3

Correcting any damage resulting from an invasion by a computer virus program is more expensive than preventing the damage.

Being compared: Correcting vs Preventing

Quality: Invasion by a computer virus

Winner: Correcting I am a bit confused on the quality part for this question, could anyone help?

For question 3, why is black holes not considered the object?

Do you recommend taking a first diagnostic PT before tackling the lessons?