Using R Studio

Given the data frame as defined below df <- data.frame(A = c(1, 2, 3), B = c(4, 5, 6), C = c(7, 8, 9)) How do you select the the vector c(7, 8, 9) using the following four approaches

(1) Using the [[ ]] operator once and only once

(2) Using the [ ] operator once

(3) Using the $ operator

(4) Using a column vector name without using the $ operator