若$\sqrt{a}$和$\sqrt{-a}$都有意义，则a的值是(        )                                     

阅读文段，回答问题。

甲文：《孙权劝学》

       初，权谓吕蒙曰：“卿今当涂掌事，不可不学！”蒙辞以军中多务。权曰：“孤岂欲卿治经为博士邪！但当涉猎，见往事耳。卿言多务，孰若孤？孤常读书，自以为大有所益。”蒙乃始就学。及鲁肃过浔阳，与蒙论议，大惊曰：“卿今者才略，非复吴下阿蒙！”蒙曰：“士别三日，即更刮目相待，大兄何见事之晚乎！”肃遂拜蒙母，结友而别。

乙文：《赵襄王学御》

       赵襄王^①学御于王子期，俄而与子期逐^②，三易^③马而三后。襄王曰：“子之教我御，术未尽也。”对曰：“术已尽，用之则过也。凡御之所贵，马体安于车，人心调^④于马，而后可以追速致远。今君后则欲速臣，先则恐逮于臣。夫诱^⑤道争远，非先则后也。而先后心皆在于臣，尚何以调于马?此君之所以^⑥后也。”

      【注释】①赵襄王：赵襄子。御：驾车。王子期：古时善于驾车的人。②逐：追赶，这里指驾车赛马。③易：更换。④调：协调。⑤诱：引导。⑥此……所以：这就是……的原因。

In order to safeguard（保证）the education of all Hope Chinese Scholl（HCS）students and the normal running of the school, the following rules should be followed:
★ Be quiet in classroom and hallways. Do not run or shout in classrooms or hallways.
★ Do not drink or eat in classrooms or hallways.
★ Stay in the school during school time as well as rest time. Do not leave the school or play out of the school area during school time. This includes rest time.
★ Do not scribble on any desks or walls.
★ Be respectful（尊敬）to teachers and parents.
★ Help teachers and other students to keep the classroom clean. Do not move desks, chairs, and other furniture in the classroom without the teacher's instructions.