交感缩血管纤维分布最密集的是（）
A.皮肤血管
B.冠状血管
C.骨骼肌血管
D.胃肠道血管
E.脑血管
参考答案：A
The meeting we had ________ to _______ us finally. [ ]
A. looked forward; disappointing
B. been looked forward; disappointed
C. looked forward; disappointed
D. been looked forward; disappointing
对作物杂种优势的遗传成因都有哪些解释？你对这些解释有什么看法？
