A.
My daughter was doing this at about that age as well. If we went into her room, she'd go back to sleep as long as we stayed there. We finally decided that wasn't a good method, because we didn't want her waking up at midnight and thus having to sleep the entire night in her room, so we let her cry for a few minutes. The first night, she woke at about 5 and cried for 10 minutes. The next night, she cried for about 5 minutes. The next night was about 3 minutes, and then she started sleeping until 7 again.
I hate hearing her cry, but she really needed to settle herself back down to go to sleep. And once she understood that, she slept much better.
Until she started climbing out of her crib and we had to put her in a toddler bed, but that's a whole different post... ;-)