人造空气（氧气和氦气的混合气体）可用于减轻某些疾病或供深水潜水员使用。在标准状况下，5.6 L“人造空气”的质量是2.4 g，其中氧气与氦气和体积比是______ ，氧气的质量是______。

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