上消化道为（）
A.从口腔到胃的一段消化管
B.从口腔到十二指肠的一段消化管
C.从口腔到空肠的一段消化管
D.从十二指肠
参考答案：B
The retired professor, _____ in his new book in the past three years, hasn’t realized the decline in his health.
A．involved
B．being involved
C．having involved
D．to be involved
受体
