下列属于人体第三道防线的是
A：皮肤和黏膜
B：血液和消化液
C：免疫器官和淋巴细胞
D：溶菌酶
一 They refused to go on a picnic on such a windy day.
一 It doesn't matter. You can do it next time.
一 How much are the two packets of ____ (battery)?
一 Ten yuan, please.
已知黑斑蛇与黄斑蛇杂交，子一代既有黑斑蛇，又有黄斑蛇；若再将F1黑斑蛇之间交配，F2中既有黑斑蛇又有黄斑蛇。下列结论中正确的是（ ）
