Which is correct?
If you don’t _______ your data effectively, you risk losing it.
b. back up
Back up is correct when it serves as a verb; backup is a noun.
By the way, the same rule holds for other words that are sometimes separate and sometimes used as a compound: When you WORK OUT you listen to your WORKOUT playlist. (Yes, I am yelling — that one drives me crazy.)
Grace Fleming covers Confusing Compounds for About.com.
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