Nio hit its highest monthly vehicle-delivery mark of the year in April.Electric vehicle (EV) sales have been slowing around the world. EV makers have even been cutting prices in China, the world's largest EV market, to spur sales. But Nio (NIO 11.65%) just surprised investors with a strong month in April, and the stock is spiking as a result.Nio American depositary shares (ADS) were trading higher by 13.2% as of 3:32 p.m. ET after the report. That move has added to a strong rally seen in the shares over the ...