Viagra,a Medication That Medically Allows for Improved Erectile Function,acts Primarily

Viagra,a medication that medically allows for improved erectile function,acts primarily by

A) increasing the level of dopamine,which is associated with sexual arousal.
B) relaxing smooth muscles allowing increased blood flow to the penis.
C) stimulating the amygdala in the limbic system.
D) inhibiting the response of the serotonin system.