Plugging a gap in the Great Firewall, China on Monday began requiring internet app stores to register with the state. China has long censored websites, barring outlawed content such as pornography, the promotion of illegal activity including terrorism and “rumors,” a term regulators often apply to antigovernment statements, according to The Wall Street Journal. But apps create a special challenge for government censors, experts say, because they often incorporate a wide variety of functions and serve as platforms for users to exchange information, making them harder to oversee. They are also multiplying quickly. The Cyberspace Administration of China, which announced new regulations on content distributed by apps in August, followed up Friday by notifying app stores that starting Monday they must register with the CAC.