I don't understand the use of units in physics EMPA past paper?

So here is the issue

AQA June2014 6X Task 2

Q1.
(a) x in mm
(b) l in mm
(c) (l+x)^1/2 - l^1/2 in m^1/2

I understand why the units for part (c) are square rooted, but why have theychanged to metres when the other values (a) and (b) were millimetres in themark scheme?

Paper: http://filestore.aqa.org.uk/subjects/AQA-PHA6B6X-PM2-JUN14.PDF
Mark scheme: http://filestore.aqa.org.uk/subjects/AQA-PHA6B6X-WS-MS-JUN14.PDF

Any help greatly appreciated!

Because the SI base and derived units for frequency, time and length are Hz, seconds and metres.

Because the SI base and derived units for frequency, time and length are Hz, seconds and metres.

So it is because you are comparing it to Frequency that you have to use metres?

Yes, because you can easily make mistakes and conversion errors when using the formula's without converting to standard units first.

For instance frequency = 1/wavelength

Using metres makes life easy.

Use mm and the answer will be x1000 in error, use cm and the answer will be x100 in error without using the correct conversion factor for each.