In the following passage some of the words have been left out. Read the passage carefully and choose the correct answer to each question out of the four alternatives and fill in the blank.

The Solar System has been a complicated wonder for the astronomers. This is a__(21)__to which we may never have the exact answer. Man has wondered__(22)__the age of the Earth__(23)__ ancient times. There were all kinds of__(24)____that seemed to have the___(25)___But man could not begin to think about the question scientifically until about 400 years ago when it was proved that the Earth revolved round the Sun and the Earth was a part of our Solar System, then scientists knew where to begin. 

Q:

Fill at the option (21).

• 1
  problem
• 2
  matter
• 3
  question
• 4
  query