Concept:First-generation computers used vacuum tubes and were large, but they offered faster calculation than earlier mechanical machines.
Explanation:First-generation computers were not portable, so portability is not an advantage.
They produced a lot of heat, so heating effect is not an advantage.
They were not specially secure, so burglar-proof is not an advantage.
Their main strength was faster processing speed compared to earlier mechanical calculating devices.
Thus, the correct advantage is processing speed.
Answer:A. Processing speed