请将下面这些合同语句翻译成中文,注意翻译时用语的准确和严密性。1.This Contract can only be altered, amended or supplemented in accordance with documents signed and sealed by authorized representatives of both parties.__2.合格投资者(QFII)必须遵守中国的法律法规和其他有关规定。__3.有下列情形之一的,合同无效。__4.If the seller fails to observe and comply with any of its obligations then the Buyer shall have the following rights.__