下列事件是必然事件的是[ ]

A.抛掷一枚硬币，四次中有两次正面朝上

B.打开电视体育频道，正在播放NBA球赛

C.射击运动员射击一次，命中十环

D.从1，3，5，7四个数中任选两个数，这两个数的和为偶数

A chart projection depicting the poles and a small area on either side of a connecting meridian，that is sometimes used for star charts，is the（）.

A.azimuthal gnomonic projection

B.Lambert conformal projection

C.transverse Mercator projection

D.polyconic projection