How long have you been collecting shells?与How long have you collected shells?的区别

How long have you collected ...这是现在完成时,强调这件事现在已经完成了,问已经收集贝壳收集了多久,暗示接下来不收集了,或不收集的可能性很大.How long have you been collecting ...表示收集贝壳的事,从过去收集...