Consider the following statements regarding powder metallurgy:
1. Refractory materials made of tungsten can be manufactured easily.
2. In metal powder, control of grain size results in relatively much uniform structure.
3. The powder heated in die or mould at high temperature is then pressed and compacted to get desired shape and strength.
4. In sintering, the metal powder is gradually heated resulting in coherent bond.
Which of the above statements are correct?
(a) 1, 2 and 3 only (b) 1, 2 and 4 only
(c) 2, 3 and 4 only (d) 1, 2, 3 and 4