雨伞的中截面如图所示，伞骨AB=AC，支撑杆OE=OF，AE=$\frac{1}{3}$AB，AF=$\frac{1}{3}$AC，当O沿AD滑动时，雨伞开闭，问雨伞开闭过程中，∠BAD与∠CAD有何关系？说明理由．

    This is my school. There are many trees and flowers in it. The teaching building is big and tall. There is a big playground in our school. After 4:30 in the afternoon, many students come to the playground to play football and some students play table tennis in the table tennis room.
There are 1,200 students in 25 classes in our school. The students all like to study. The teachers in our school are very good. They help the students to study and the students like them, too.

上海合作组织成员国政府首脑（总理）理事会第十四次会议于2015年12月在郑州市成功举办。上合组织的合作最先起源于安全事务，目前进一步推动区域内交通、能源、通信基础设施互联互通和自由贸易安排，为成员国经济发展提供更大助力。由此关于上合组织表述正确的是（    ）

①是一个新型的区域合作组织

②开创了结伴不结盟的新型国际关系

③合作的领域不断拓宽

④由经济一体化走向政治一体化