I believe the answer is: testosterone
testosterone is highly correlated due to the libido that humans have.
Even if the level of testosterone increase within the ratio of 10% and below, it would improve the likelihood of someone engaging in sexual activity to almost twofold.
(A. <span>Operant conditioning-------------------------</span>
The correct answer should be C. contact
Intimacy and involvement come at later stages where the relationship has escalated more. Contact is the part of initiation.