题目

When we donate（捐献）blood, a small amount is usually taken in advance for at least ABO and Rh systems typing. If you are O+, the O is your ABO type and the + is your Rh type. It is possible to be A, B, AB, or O as well as Rh + or Rh-. The ABO system was discovered by Karl Landsteiner in the early 1900s. He was awarded the Nobel Prize for his achievements. There are four basic types of blood in the ABO system: A, B, AB and O. Everybody is born with one of these four types of blood. We get blood type, just like hair color and height from parents. Because of the substances （物质） con18．过点A（-3，3）和点B（6，3）的直线一定（　　）A．垂直于x轴B．于y轴相交但不平行于x轴C．平行于x轴D．平行于y轴