挤压井时，要先泵人隔离液，压井液挤至油层顶界以上（），要防止将压井液挤入地层，造成污染。
A、20m
B、50m
C、100m
D、150m
参考答案：B
A: Aren’t we supposed to have the meeting begun at 9 Where is the chairB: ().
A. You know our director. He is always behind schedule.
B. I know it’s very later
C. No. I’ll have it tomorrow, not today.
D. See you then.
简述铸坯纵裂产生的原因。
