胸骨中上段后方疼痛，向左肩部放射，最可能的病因为（）
A.胸膜疾病
B.心绞痛
C.肋间神经病变
D.食管炎症
E.肋骨骨折
参考答案：B
生态平衡是指生态系统中的各种生物( )。
A．数量不变
B．比例不变
C．数量和所占比例保持相对平衡
D．数量暂时平衡
—I don’t know ______. Can you tell me? —Yes, at 3 tomorrow afternoon.
A．when we had a meeting
B．when did we have a meeting
C．when we will have a meeting
D．when will we have a meeting
