下列各大洲中,地形以平原为主的是( )
A:亚洲
B:北美洲
C:欧洲
D:大洋洲
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70×8 =____ 405 × 2 =____ 50÷5=____ 6×110=____
420÷7=____ 39÷3=____ 400÷4=____ 300×5=____
291×0=____ (20+30)×5=____ 4+2×8=____ 70-6×0=____