Table in R showing mean values from dataframe

Question

I have a data set that is formatted similar to this:

A | B | C
---------
0 | 1 | 2
0 | 2 | 3
1 | 5 | 9
1 | 2 | 11
2 | 6 | 7
2 | 3 | 4
3 | 8 | 10

The data is in a dataframe and I'm currently looking to create a table that shows the mean of B and C for each unique value of A.

The resulting table would look something like this:

A |       B       |      C
--------------------------------
0 | mean(B for 0) | mean(C for 0)
1 | mean(B for 1) | mean(C for 1)
2 | mean(B for 2) | mean(C for 2)
etc

Is there a function that can do this in R? Or is there a different recommended way?