Yingli Green Energy Holding Co., once the world's biggest solar manufacturer, plunged the most in more than seven months after signaling it may be teetering toward bankruptcy. Yingli declined 21 percent to $3.60 at the close in New York, the most since Sept. 29. That followed an 8.1 percent drop Friday after the Chinese solar company acknowledged " substantial doubt as to its ability to continue as a going concern."