What is the complementary color of blue?

Which of the following is not a secondary color?

$A$ bifocal lens consists of both concave and convex lenses. Bifocal lenses are required by a person suffering from $.............$.
$P$ - Myopia (nearsightedness)
$Q$ - Hypermetropia (farsightedness)
$R$ - Presbyopia

What would have been the colour of the earth's sky if there were no atmosphere?

$A$ student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw a ray diagram for the correction of this defect.

For a person suffering from myopia,the image of the object is formed in front of the retina.