An Ad Agency Is an Expert in Placing Advertisement Messages

An ad agency is an expert in placing advertisement messages in different media,like television,radio,magazines,and digital/social media.It effectively mixes and matches all of these media to make sure that an advertiser's message makes a positive impact on the target audience.It is also an expert in creating the schedule in which these messages will appear.This ad agency is working effectively as a: 

A)  media bidder. 
B)  media planner. 
C)  media vehicle. 
D)  media class.