已知△ABC为等腰三角形，①当它的两个边长分别为8cm和3cm时，它的周长为（    ）cm；②如果它的周长为18cm，一边的长为4cm，则腰长为（    ）cm．

It is our duty     _   our hometown clean and beautiful. We must do something for it.  [ ]

A. to keep  

B. keeps  

C. keeping