A:worked
B:had worked
C:would work
D:were working
C51+C52+C53+C54的值为( )
做重复试验,抛掷同一枚啤酒瓶盖,经过统计得“凸面朝上”的频率约为0.44,则可以估计抛掷这枚啤酒瓶盖出现“凸面朝上”的概率约为( )