China on Thursday threatened to make a "legitimate and necessary" retaliation over the United States (US) sale of US$1 billion worth of missiles to Taiwan as Beijing becomes increasingly strident over its claims to the self-ruled island.The US State Department said on Wednesday it had approved the sale of 135 air-to-ground missiles to Taiwan in a move Taipei's defence ministry said would build its combat capabilities.Democratic and self-ruled Taiwan lives under constant threat