镰刀状细胞贫血病是由于血红蛋白______。
A. β亚基氨基酸发生了单位点突变
B. α亚基氨基酸发生了单位点突变
C. β亚基氨基酸发生了多位点突变
D. α亚基缬氨酸发生了突变
肾上腺髓质激素分泌所受支配的神经纤维是______。
A. 交感神经节后纤维
B. 交感神经节前纤维
C. 副交感神经节后纤维
D. 副交感神经节前纤维
Goods that are returned ______ warranty should be accompanied by proof of purchase.
A. with
B. under
C. over
D. on
The Caribbean islands are still one of the most popular tourist ______ during the winter season.
A. situations
B. positions
C. directions
D. destinations