What words does the author use to emphasize the concept of outer space as an eternal frontier?

A all that has gone before was preliminary

B everlasting challenge to explorers

C the line that separates the know from the unknown

D mankind is not bound by its atmospheric

D mankind is not bound by its atmospheric envelope

The author uses 'everlasting challenge to explorers' to emphasize the concept of outer space as an eternal frontier.

Explanation:

In this question, option B is the correct answer. This is because it is emphatic and agrees with the given sentence. It is also grammatically correct. In the given sentence, the author wants to focus on the difficulties and the problems that the explorers will have to face in their expedition.

He wants to explain to us the hardships and the thrill about the outer space that the explorers are about to go through.