若向量 $\overrightarrow{a}$ ， $\overrightarrow{b}$ 满足| $\overrightarrow{a}$ |= $\sqrt{3}$ ，| $\overrightarrow{b}$ |=2， $\overrightarrow{a}$ ⊥（ $\overrightarrow{a}$ ﹣ $\overrightarrow{b}$ ），则 $\overrightarrow{a}$ 与 $\overrightarrow{b}$ 的夹角为（   ）

已知a>0，x、y满足约束条件$\{\begin{array}{c}x\ge 1\\ x+y\le 3\\ y\ge a\left(x-3\right)\end{array}$，若$z=2x+y$的最小值为$\frac{3}{2}$，则a=   （  ）

    First opened to the public in 1976,the California State Railroad Museum is one of Sacramento's largest and most popular visitor destinations.Over 500,000 people visit the museum every year,with guests traveling from throughout the world to experience this world-famous place.

    The museum consists of six original buildings.The main exhibit building,the Railroad History Museum,totals 100,000 square feet.Completed at a cost of US $16.1 million,it opened in May 1981.

    Hours: The museum is open daily (except on Thanksgiving,Christmas and New Year's Day) from 10 a.m. to 5 p.m. From May 28 to September 3,the museum will be open until eight o'clock in the evening.

    Admission: US $10 for adults,US $5 for youths aged six to seventeen;Children aged five and under are free; Cash,personal checks,traveler's checks and credit cards are accepted for payment.

    Location: The California State Railroad Museum is located at the corner of Second and “I” street in Old Sacramento.

    Parking: A limited number of spaces are available for parking on the streets near the museum.However,these spaces are not suitable for guests visiting longer than 90 minutes.All-day parking is available in the large public garage at the “I” Street entrance to Old Sacramento,for a small fee.

    Food Service: Many restaurants are located near the museum.These range from reasonably priced, family-friendly places to some of the finest dining restaurants in the city.

    Accessibility: The museum is fully accessible to people with disabilities.Certain exhibits and programs have limited accessibility,due to factors such as narrow and historic stairways.

    Photography: We welcome photography for personal use.Hand-held cameras are allowed;however,tripods are not permitted for safety reasons.A Morning for Photographers at the museum is a special event.Tripods are allowed during this event.

单句改错

①Struggled in a world of silence and darkness, Helen Keller seemed to be simple-minded.____

②The three Bronte sisters adopted this practice, Mary Ann Evans did the same.

____

③We decided to fight against this unfair system which prohibits us blacks sitting where we like.____

④Our friendship had developed quickly over the weeks that followed.____

⑤Maybe if I studied science then,I would have been able to give you more help.

____