CHIA ĐỘNG TỪ: 1, Though doctors ________________ (know) for decades that a change in diet could reduce the symptoms of celiac disease, it wasn't until the 1950s that the disease was linked to wheat proteins. 2, Mrs. Baldwin _________________ (teach) ninth-grade geography at Montgomery High before leaving to pursue a master's degree in cartography. 3, Historically, men ________________ (dominate) the field of science, and many women _________________ (not receive) professional credit or public recognition for their work. 4, Bell ____________ (be) a postgraduate student in 1967 when she _________________ (discover) the first radio pulsars—small, dense rotating stars that emit radio waves. 5, Genghis Khan _________________ (become) the first Mongol to rule over all of China, and he _______________ (found) the country's Yuan Dynasty. 6, In the 1920s, William Baer _______________ (begin) treating hospital patients' serious wounds with maggots.