What did frank lloyd wright use to unite indoor and outdoor spaces in buildings like fallingwater (edgar kaufmann residence)?

Frank Lloyd Wright use Cantilevers.

Explanation:

The house Fallingwater was built in 1935 by  Frank Lloyd Wright an American architect. This house was created as a private residence for Edgar J. Kaufmann, Sr. This house is considered to be one of the famous architected houses by Wright. It depicts the balance between nature and art, it has become a museum which attracts tourists to see its beauty.

answer; (a) he was a u.s senator for 38 years;