Assuming they attack with 6 zerglings, I'll make up a quick strategy.
1. Playing as Terran - You probably wont have enough time to make a refinery, and thus firebats, so you'll have to rely on marines.
One well placed bunker with 3-4 marines and a SCV repairing it will be fine.
2. Playing as Zerg - You have a choice here, either buildings or units. I personally prefer using sunken colonies. While your spawning pool is being built (Slow!) make two creeps, and as soon as that pool is done, make them into sunkens. Might as well make two zerglings while you're at it.
Or, you can ignore the creep completely and just make 6-8 lings.
3. Playing as Protoss - The most difficult, because it takes years to make everything. Your best choice is to make a gateway, and 2 or 3 zealots. One zealot can handle 2 lings. If you have time, make a forge and thus a cannon quickly near your nexus.
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