Two steel columns P (length $L$ and yield strength $f_y=250$ MPa) and Q (length $2L$ and yield strength $f_y=500$ MPa) have the same cross-sections and end-conditions. The ratio of buckling load of column P to that of column Q is
1. $0.5$
2. $1.0$
3. $2.0$
4. $4.0$