an usable vs a usable

Some examples from the web:

• For longer phrases (4+ words) it's not always possible to find identical examples. Please try again with a shorter phrase.

  For the very best results, we recommend that you submit your full sentence to our Quick Text editing service so a human editor can provide you with the right suggestions.

Some examples from the web:

• There were too many cracks to see a usable print with powder.
• It was fired recently, but I couldn't get a usable print off it.
• Were you able to get a usable print?
• No, and I was unable to get a usable fingerprint.
• I'll cash it when we have a usable enzyme.
• We have a usable enzyme that's ready for clinical trials.
• I was able to extract a usable profile from the remnants of the sandwich...
• Where there is a usable estimation of human exposure, the lowest level should exceed this.
• Where there is a usable estimation of human exposure the lowest level should exceed this.
• Could not find a usable proxy configuration script
• I'm having some trouble finding a usable vein.
• So without surgery, we have given Kiko the gift of a usable prosthetic appendage.
• All right, well, maybe there's a usable print.
• Now the guys in the lab were able to humpty-dumpty together a usable print.
• I was able to extract and amplify a usable profile, but the bad news is... there's no hits in CODIS.
• Nor is there any infrastructure to turn the information produced by Article 25 into a usable format for investors.
• Stabling tracks intended for use by trains complying with High-Speed Rolling Stock TSI shall have a usable length sufficient to accommodate these trains.
• I noticed there's a usable space on the other side of this wall.
• It took me half an hour to find a usable vein on you.
• So CSU was able to pull a usable print?