Choose the option which best fits gap 21.

Directions: Fill in the gaps with the most appropriate option.

Ever since  Galileo pointed his homemade optical telescope towards the heavens, astronomers---18--------------------building instruments of increasing power and sophistication ---------19--------------------to observe the universe. Hale telescope on Mount Palamar in Southern California can   detect objects a million times smaller than Galileo’s device -----21--------, they still have their limitations. With the advent of radar technology during World War 2, however, astronomy took a great leap outward. With radio antennas pointing to the night sky, astronomers far -----23-------------------their reach.