R question

If I have data like

a <- data.frame(detail=c('pure water','mineral water','wood fire'),2)
b <- data.frame(type=c('water','fire'))
a

##          detail X2
## 1    pure water  2
## 2 mineral water  2
## 3     wood fire  2

b

##    type
## 1 water
## 2  fire

c <- merge(a,b,all=T)

Ideallly, the result should be R

r <- c[c(1,2,6),c('detail','type')]
r

##          detail  type
## 1    pure water water
## 2 mineral water water
## 6     wood fire  fire