Grammar

Look at the sentences below. Choose the correct form. Sometimes two forms are possible. HELP
1)The practical means of ……………. it still lay far in the future.
2)………………. insights into space travel and rocket science was a challenging task.
3)Progress in science would eventually allow humans ………………… immortality.
4)The population would swell so much that humanity would have …………. across the universe.
5)These were the ideas that awakened Tsiolkovsky’s interest in ……………. outer space.
6)Around that time he could ……………………. his first scientific works.
7)He tried ………………………. the meaning of the cosmos for humanity and the effects that vacuum and weightlessness would have on future space travelers.
8)The manuscript also contained a sketch considered …………. one of Tsiolkovsky’s earliest depictions of a spacecraft.
9)Also in Borovsk, Tsiolkovsky started ……………… designs for airships.
10)Tsiolkovsky thought that it is more efficient …………….his theoretical engine with a mix of liquid oxygen and liquid hydrogen.
11)It is still believed …………… the most efficient of chemical rocket propellants.
12)Tsiolkovsky arrived at the combination with little hope of …………………. his theory.
13)He never attempted …………………. a rocket engine.
14)Oberth later helped …………………………. Tsiolkovsky’s name widely known.
15)In 1926, Tsiolkovsky published Plan for Space Exploration, a bold 16-step program which would let human civilization ……………………. the universe.
16)According to Tsiolkovsky, humans will be able …………………….. the asteroid belt, the solar system, and ultimately the galaxy.
17)It would allow a payload ………………………….. indefinitely through the vacuum.
18)He proposed ……………………… graphite rudders to steer a rocket in flight.
19)Tsiolkovsky is remembered for …………………….. in the dominance of humanity throughout space.
20)In the years following the Bolshevik Revolution, it was important ……………….the superiority of the Soviet system.