Joe Biden was seen snapping at a White House aide in his latest gaffe as he met world leaders for the final time as president. The 81-year-old abruptly barked “Who’s next?” while introducing the “Quad” leaders of Australia, India and Japan. Biden hosted the event on Saturday night in Wilmington, Delaware to discuss the Quad’s efforts to beat cancer. Following his introductory speech, Biden said: “So I want to thank you all for being here. And now, who am I introducing next?” After an awkward silence, the US president demanded: “Who’s next?” One of his aides could be seen making a hand gesture …