How to retrieve data by following conditions?

How to retrieve data by following conditions?

+--------+------+---------------------+
| userId | Cmd | Time |
+--------+------+---------------------+
| 1 | A | 2013-06-21 13:15:08 |
| 1 | B | 2013-07-31 15:58:53 |
| 3 | A | 2013-07-31 16:01:42 |
| 4 | A | 2013-05-01 11:05:08 |
| 4 | B | 2013-07-31 16:06:32 |
| 6 | B | 2013-07-31 17:16:08 |
| 7 | A | 2013-04-06 17:20:36 |
| 3 | B | 2013-04-15 06:26:14 |
| 7 | B | 2013-07-31 14:53:49 |
| 6 | A | 2013-07-31 09:56:27 |
+--------+------+---------------------+
How to get userIDs where their last Cmd is B?