-- Such beautiful flowers! I can't decide _______ for my mum.
-- For Mother's Day, it can't be better to take some carnations (康乃馨).
A：to choose which
B：how to choose
C：which to choose
D：when to choose
下列溶液中的Na+与50mL 1mol•L﹣1 Na3PO3溶液中Na+的物质的量浓度相等的是（ ）
关节活动起来非常灵活，与之有关的结构特点是关节结构里有（ ）
晓雯在二胡演奏时，用弓拉动琴弦，使琴弦____而发声；表演中不断用手指控制琴弦长度，这样做是为了改变声音的____。
