Not until the early years of the 19th century ________ what heat is.

A. man did know　　　　 B. man knew

C. didn’t man know　　　 D. did man know

Not until the early years of the 19th century____ what heat is.

A.    man did know B. man knew      C.didnot man know     D. did man know

Not until the early years of the 19th century______ what heat is.

A. man did know       B. man knew         C. didn’t man know    D. did man know

Not until the early years of the 19th century______ what heat is.

 Not until the early years of the 19^th century______ what heat is.

A. did man know     B. man had known      C. man knew      D. had man known