Assume there is a population of firms that either make high quality products or low quality produces. The only problem is consumers can't tell which are the high quality firms and with are the low quality ones. For the high quality firms to send a signal that is effective at separating them from the low quality group, the signal must: a. be credible and hard for the low quality group to mimic b. include a certification from the government c. have a celebrity endorsement d. all of the above