The chief engineer of the high-speed railway ________ Shanghai to Beijing always devotes most of his time ________ his work.
A:connecting; to
B:connects; to
C:connected; on
D:connect; on
—How old are you? — .
20×3=____
23×3=____
关于磁场说法正确的是( )
下列关于有氧呼吸产生的C02和H2O,正确的是( )