“绞釉”是指（）。

A.在牙颈部釉柱排列成水平状

B.釉柱向窝沟底集中

C.釉柱自釉牙本质界生牙表面排列不成直线

D.釉柱近表面1／3较直，内2／3弯曲

E.釉柱在切缘及牙尖处绞绕弯曲

Passage Three

What would happen to some countries if tourism stopped

A．Their economies would be ruined.

B．It would keep their economies going.

C．There would be no more environmental pollution there.

D．People have no chance to travel there.