Which of the Following Best Describes the Mechanism of an S_N2

Which of the following best describes the mechanism of an S_N2 reaction?

A) The nucleophile attacks and the leaving group leaves in a concerted fashion.
B) The nucleophile forms a bond with the electrophile first,followed by the loss of a leaving group.
C) The leaving group leaves first,followed by the attack of a nucleophile.
D) The electrophile attacks and the leaving group leaves in a concerted fashion.