Which statement concerning GPS is TRUE（）.

A.It cannot be used in all parts of the world

B.There are 12 functioning GPS satellites at present

C.It may be suspended without warning

D.Two position lines are used to give a 2D fix

有关甲型肝炎，下列哪项是错误的。（）

A、潜伏期及发病早期排毒量答

B、易发生食物和水型暴发

C、采取免疫预防为主的综合措施

D、无慢性化倾向

E、在甲肝高流行曲，甲肝发病主要集中于低年龄人群