1. The basic runway length should be increased at the rate of 7 percent per 300 m rise in elevation above the mean sea level
2. The basic runway length after having been corrected for elevation should be further increased at the rate of 1 percent for every 1°C rise in airport reference temperature above the standard atmospheric temperature at that elevation
3. The runway length after having been corrected for elevation and temperature should be further increased at the rate of 20% for every 1 percent of effective gradient
Select the correct statment

A. 1 and 2 are correct
B. 2 and 3 are correct
C. 1 and 3 are correct
D. 1,2 and 3 are correct