use ifelse to create new column in r

My code is:

data$E<-ifelse(data$D==data$B, "b", ifelse(data$D==data$C, "c", "unknowwn"))

result:

  A   B   C   D   E
1 16  16  NA  16  b
2 20  NA  20  20  NA
3 24  NA  NA  24  NA

But what I want is:

  A   B   C   D   E
1 16  16  NA  16  b
2 20  NA  20  20  c
3 24  NA  NA  24  unknowwn

Dose anyone know how to solve this problem? Thanks!