天门冬、威灵仙中的木质部中有一种厚壁细胞，末端较尖，壁上有具缘纹孔，开口呈双凸镜状或缝状称韧型纤维。( )
参考答案：对
– You seem to know much about the city.
– That’s true. I _______ it four times in the last three years.
A．visited
B．am visiting
C．have visited
D．will visit
下列各项，不属艾滋病典型表现的是
A．口咽念珠菌感染
B．长期发热
C．头痛，进行性痴呆
D．皮肤黏膜出血
E．慢性腹泻
