It was ______ the 19th century_____ heat was considered to be a form of energy.
A.not until; that
B.until; that
C.not until; when
D.until; when
答案:A

It was ______ the 19th century_____ heat was considered to be a form of energy.
A.not until; that
B.until; that
C.not until; when
D.until; when
答案:A