Fuses prevent circuit overloads; nails do not.

Steps:

• The fuse blows due to excessive current from the kettle, indicating a fault like a short circuit.
• Replacing with a nail bypasses the fuse's protective function, as nails conduct electricity without melting.
• Without protection, high current can flow unchecked through the wiring.
• This overloads the house wiring, generating heat that may ignite a fire.

Why A is correct:

• Fuses are designed to melt at a specific current (per Ohm's law, I = V/R; high I causes heat), protecting wiring; a nail lacks this limit, allowing dangerous overloads.

Why the others are wrong:

• B: Fuse type doesn't affect water volume needed to boil.
• C: Boiling point of water is fixed at 100°C at standard pressure, independent of electrical issues.
• D: No real fuse exists with the nail, so blowing doesn't apply.

Final answer: A