如图所示为植物体的结构层次，属于器官的是（　　）

马克思能够认识到无产阶级是改造旧世界、建设新世界的伟大力量,主要是由于他(  )

不属于我国发展中面临的挑战的是（　　）

—Did you have a good time?

—It couldn't be worse. We were held up on the expressway for three hours. On the way back, we found the car got a f____ tyre, and the spare was, too.