螺纹连接时的防松动措施不包括（）
A.双螺母
B.止动垫圈
C.止动垫片
D.千斤顶
参考答案：D
An investor purchases 200 shares of Merxx on margin. The shares are trading at $40. Initial and maintenance margins are 50 percent and 25 percent. If the investor sells the stock when the price rises to $50 at year-end, the return on the investment would be closest to:（）
A. 50.00%.
B. 18.75%.
C. 25.00%.
青年男性患者，无明显外伤史，出现视力下降，伴明显玻璃体积血，应考虑哪种疾病( )
A．糖尿病视网膜病变B．中央静脉阻塞C．视网膜静脉周围炎D．视网膜脱离E．视盘血管炎
