2个月至儿童期引起化脓性脑膜炎的常见细菌是
A．大肠杆菌B．副大肠杆菌C．金黄色葡萄球菌D．流感嗜血杆菌E．白色葡萄球菌
参考答案：D
An analyst does research about cash flow classification. The initial cash payment for an equity security is considered an investment cash flow. The security is least likely to be classified as:（）
A. held for trading.
B. held to maturity.
C. available for sale.
简述病例对照研究的原理及优缺点。
