@gongji, the A and B connectors are to distinguish a host from an endpoint. The A connector plugs into a host and the B connector plugs into an endpoint. So, the cable between your PC and your printer has an A connector on the end going to the PC and a B connector going to the printer. This helps keep the signal polarities correct as well as preventing someone from trying to use the printer as a USB host.
In fact, I dont think any USB cable has the A connector on both ends. If it did, somone could really mess things up by trying to connect two hosts together.
Your flash drives have only an A connector because, in essence, the functional equivalent of the cable with its B-end is an integral part of the flash drive, so only the A end of the "cable" is actually used.
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