You are developing an internet-based training solution for remote learners. Your company identifies that during the training, some learners leave their desk for long periods or become distracted. You need to use a video and audio feed from each learner's computer to detect whether the learner is present and paying attention. The solution must minimize development effort and identify each learner. Which Azure Cognitive Services service should you use the following requirement? "From a learner’s video feed, verify whether the learner is present."