It’s a bomb! A mountain! A horror movie and a treadmill to hell! To doomsayers, China’s $28 trillion pile of public and private debt is a threat to the global economy. Or maybe it’s just a manageable byproduct of the boom that created the world’s second-biggest economy. Either way, the buildup has been breathtaking, with borrowing quadrupling in seven years by one estimate. (China doesn’t give a complete tally). Weaning the nation off that debt without intensifying an economic slowdown is tricky. Because China is a key driver of global growth, the solution is everybody’s concern. Propping up borrowers to prevent defaults is one approach. That could leave the country mired in bad debt and susceptible to years of stagnation.