What century is this?

Which is correct?
Modern financial bookkeeping practices date back to the ___ century, possibly about 1250.
a. 11th
b. 12th
c. 13th

Explanation:
A lot of people have trouble with this one and I have to admit that I’ve had times where I had to stop and think whether the 1700s were the 16th century or the 18th century (I knew they weren’t the 17th century. Noooooo, that would be too easy.)

Here’s one way to think about it:
The first century began in year one of the common era (CE, which is the PC AD); the second century began after the end of year 100. So, if you follow that along the line, you’ll get to the 1800s representing the 19th century, the 1900s representing the 20th century and the 2000s representing the 21st century. Anyway, the century is always one number ahead of the first two numerals in the year in question.

Or, on the other hand, you could just remember that this is the 21st century and then work it back from there.

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