r - how to inner join two tables on selected columns in R

Question

df1 <- data.frame(a = c(1:5), b = c(6:10), c=c(6:10))
df2 <- data.frame(a = c(1,3,5,7,9), b = c(16:20), c = c(16:20), d= LETTERS[1:5], e= LETTERS[6:10])