When SN2ed speaks, it is best to listen. In all seriousness, it sounds like you should do a bit of research into how to prep for the MCAT. Generally diagnostics are considered worthless unless using them solely for practice. As SN2ed said, TBR and TPRH Science Workbook are full of great passages that provide excellent practice. I don't know if getting both is necessary, though. That is, unless you wish to test yourself over multiple topics at one time, which TPRH seems to do a bit more (not all that much). Either resource is great, and can be found for a reasonable price on the For Sale forums on SDN. Good luck!