Some of the More Successful Advertising Agencies,like TBWA and Wieden

Some of the more successful advertising agencies,like TBWA and Wieden & Kennedy,attribute their excellent creative work to the fact that:

A) they have clients who are risk averse.
B) they have clients who are willing to take creative risks.
C) they have clients who never become involved with or interfere in the creation process.
D) their clients keep them in check and don't let them try creative approaches that are excessively different.
E) their relationship with the client is significantly more important than any creative work.