[单选题]39、 By default, after how many DPD R_U_THERE packets sent without receiving a DPD_R_U_THERE_ACK response is a peer declared down and a session deleted?默认情况下，向对方发送多少个DPD R_U_THERE数据包后，却没有收到来自对方的DPD_R_U_THERE_ACK响应，就可以宣告对方down并删除会话?
A. 3
B. 4
C. 5
D. 6