Multiple submission is the game these days, and agents know it. Whether the offer comes via email or phone, convey your excitement about the opportunity to work together then say you’ve multiply submitted and would like a week or so to respectfully notify the other agents before you accept any offer. That’s professional, and a fellow professional will respect it. Authors are emotionally invested in their work and can lose sight of representation being a business partnership. It’s O.K. to be thoughtful about inking a deal. In your email subject line to the other agents, write “Submission follow-up: representation offer received.” That may prompt them to review your submission. I just watched this play out for a writer and all agents were responsive and gracious. If you get a preferable offer, tell the first agent that you received another offer and that the decision was hard but you’ve finally made the call to go with the other agent. Everyone has been respected in this exchange, and you’ve honored your career in the process.