在调节放松状态下，平行光线经眼屈光系统后聚焦在视网膜前，这种屈光状态为( )

A．远视

B．近视

C．散光

D．弱视

E．正视

As viewed from space, the Earth’’s distinguishing characteristics are its blue waters and white clouds. Enveloped by an ocean of air consisting of 78% nitrogen and 21% oxygen, the planet is the only one in our solar system known to harbor life. Circling the Sun at an average distance of 149 million km (93 million miles), the Earth is the third planet from the Sun and the fifth largest planet in the solar system. Its rapid spin and molten nickel-iron core give rise to an extensive magnetic field which, coupled with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Most meteors burn up in the Earth’’s atmosphere before they can strike the surface. The planet’’s active geological processes have left no evidence of the ancient pelting it almost certainly received soon after it was formed. The Earth has a single natural satellite — the Moon.

Approximately how much of the Earth’s atmosphere is nitrogen

A．One-fourth.

B．One-half.

C．Three-fourths.

D．All of it.