A：Because he got a headache.
B：Because it's too noisy outside.
C：Because he stayed up late doing his homework.
— Is this your mother?
— _______. She's my aunt.
草本植物的茎不能逐年加粗是因为茎内没有____．
用固体硝酸钠配制溶质质量分数一定的溶液时，正确的操作顺序是（ ）
