请选择ABCD/y8E,U,{9p,I/h)@3lNot until the early years of the 19th century ___ what heat is.3v9c)N)M'G(E:N
A. man did know3v9c)N)M'G(E:N
B. man know3v9c)N)M'G(E:N
C. didn't man know3v9c)N)M'G(E:N
D. did man know3v9c)N)M'G(E:N