Which pair amount the following solid state welding processes

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Which pair amount the following solid state welding processes uses heat from an external source
P. Diffusion welding
Q. Friction welding
R. Ultrasonic welding
S. Forge welding

1. P and R
2. R and S
3. Q and S
4. P and S

Correct Option: D

Diffusion & forge welding uses external heat source.