Laine Higgins wanted to earn some extra cash. So she applied for a job as a dog walker in New York City with one of the many on-demand dog-walking services. And was denied. "I got one question wrong on the safety test, and I think what did me in was all of the questions they have about harnesses. So to their credit, they're very thorough." Higgins isn't alone. According to her report in the Wall Street Journal, the Seattle based pet-care app Rover accepts just 15 percent of applicants. For Wag!, the acceptance rate is 5 percent. Marketplace Weekend's Lizzie O'Leary spoke with Higgins about the U.S. dog walker market and the challenges of becoming part of the pack.