Four sentences are given. You need to find if they are grammatically correct or incorrect and mark the answer accordingly.

P. Selection of personnel for airports should be done meticulously.

Q. No sooner had I given a graceful reply to the letter when I began to realize what was expected of me. 

R. The real truth is that science is not man's nature, it is mere knowledge and training.

S. Environmental issues are currently at the centre of a heated debate.