Consider the following work materials:
1. Titanium 2. Mild steel
3. Stainless steel 4. Grey cast iron
The correct sequence of these materials in terms of increasing order of difficulty in machining is
(a) 4, 2, 3, 1 (b) 4, 2, 1, 3
(c) 2, 4, 3, 1 (d) 2, 4, 1, 3