Sorting data between rows in R

Hypothetical data:

 a <- c(400,500,600,700,100,600,700,100)
 b <- c(2,2,1,2,2,1,2,1)
 c <- c('NA','R','NA','G','NA','R','NA','G')
 data <- data.frame(a,b,c)

Output:

    a b  c
1 400 2 NA
2 500 2  R
3 600 1 NA
4 700 2  G
5 100 2 NA
6 600 1  R
7 700 2 NA
8 100 1  G

You can easily subset if it is in the same row:

subset(data, b== '1' & c =='R')

Output:

    a b c
6 600 1 R

My question is how do I subset between rows? For example, how do I find all values of c = 'R' when b = '2' on the above row?

    a b  c
2 500 1  R
6 600 1  R