Beside steel, what was the other breakthrough that allowed buildings to grow so tall?

Elevators and lack of city space.  Since there was less real estate available on land, they had no other choice but to build upwards.  

a)scientists

explanation:

the national defense education act had two main goals. the first one was to provide personal that was oriented to defense, including scholars, engineering students and so on. the other goal was to provide financial assistance for the students who would be enrolling at colleges and universities in the 1960s.

answer; i believe that the correct answer is (c) arctic tundra;