A:if they were interested in dinosaur
B:when was Albert Einstein bor
C:what they will do with the computer
D:how many trees they have planted
某单位用3.2万元购买了一台实验仪器,假设这台仪器从启用的第一天起连续使用,第n天的维修保养费为n+4910n∈N*元,若使用这台仪器的日平均费用最少,则一共使用了____ 天.
下列各句中,没有语病的一项是( )