Answer:
Difficulty
Explanation:
Space exploration was the last thing the world could have thought of dabbing into because the idea of moving away from the earth and flying in the sky wasn't even an easy one for man, hence the difficulty attached with going into other planets was a big success.
Many trials were made before the eventual success experienced, this many difficulties man overcame is what is meant by barrier in this sense.
b and d becuaseregard is the last one and respect is the second one.
Seeds begin as a very small object or device, just like our courage. We only need a tiny little seed of courage for it to grow, and once you give that little courage it continues to grow.
