题干
Look! David is helping his mother _______ the room.
A：is cleaning
B：cleaning
C：cleans
D：clean
答案(点此获取答案解析）
D
Look! David is helping his mother _______ the room.
A：is cleaning
B：cleaning
C：cleans
D：clean
Na2CO3溶液950mL，配制时应选用的容量瓶的规格和称取Na2CO3的质量分别是