下列有关水的叙述中，错误的是（  ）

①参与运输营养物质和代谢废物的水为自由水；

②生物体内的化学反应离不开水；

③水是细胞结构的组成成分之一；

④人体细胞内水的存在形式为结合水和自由水；

⑤自由水与结合水的比例与新陈代谢的强弱关系不大；

⑥不同细胞内自由水与结合水的比例相差不大

    Since a university education in the United States ____(be)very expensive, most students work in addition to studying. Students ____(usual)work part-time, but some students take full-time jobs.

    A typical job for a students would be working ____a waiter or waitress. Other typical off-campus jobs would include being a clerk in a store or delivering pizza. Sometimes students also find jobs on campus such as cleaning, working in a cafeteria or library or answering ____(telephone). Most student jobs are entry-level and low paying, but have flexible schedule____(allow) students to attend classes.

    Sometimes students also hold more responsible positions such as managers, or if they are lucky, positions in their area of study,____they hope will help them find employment after they graduate.

    ____double responsibilities of working and studying result in a very busy,____ (stress)life for some students.____, it is the only way for many people to finance their college educations. Some Americans believe that the work experience is good for the students ____(they)since it gives them a taste of “the real world” outside the school.

设l，m，n表示三条不同的直线，α，β，γ表示三个不同的平面，给出下列四个命题：

①若l⊥α，m⊥l，m⊥β，则α⊥β；

②若m⊂β，n是l在β内的射影，m⊥l，则m⊥l；

③若m是平面α的一条斜线，A∉α，l为过A的一条动直线，则可能有l⊥m且l⊥α；

④若α⊥β，α⊥γ，则γ∥β

其中真命题的个数____ ．