Attempt, when starts

Yes it is - it is not the requirement of law that A should have dealt the fatal blow or even backlifted his hand to do so - in order for his act to amount to attempt to murder. It need not be the penultimate act. It should be something towards commission of offence (which it was) - after preperation is over - and should reflect the mens rea of the preparator of the attack, it was foiled due to a supervening cause (that is apprehension) a third party factor beyond his control with he doing whatever he could. Here it satisfies all the esentials. However proximity of act to actual consummation is not entirely out of question : If B was running say about 500 meters ahead - it cannot be called as an attempt to murder.  Hence whether act amounts to attempt or not is highly circumstantial. In this where the person was caught - alongwith the totality of circumstances would be relevant 

Rightly explained.