哪句才對?謝謝!!!!!

The unit of length is metres

The third one is the correct answer.
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The unit of length is meters.

The unit of length is metres.

B. The length unit is metres.
B. 長度單位是米。

Hi,
A chain is a unit of length; it measures 66 feet or 22 yards (20.1168m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in England and in some other countries influenced by British practice.
http://en.wikipedia.org/wiki/Chain_(unit)
You may say:
1) A meter is a unit of length. ( Not only one unit of length)
2) A unit of length is a meter.
3) The meter is a unit of length.
4) The unit of length is a meter.
Note: It is " a unit", NOT " an unit".
It is because the word" unit" doesn't start with a vowel sound.

Here" a unit of legnth" refers to " a meter".
" meters" is the PLURAL of ' meter".
It would be confusing here.
If you say " The unit of length is METERS", then
you also mean " XXXX is a( singular) meters( plural) long <- the unit here"
which is WRONG.
Also, there are some units of length, in this case you may
make it clearer by saying:
1) One of the units of length is a meter.
OR
2) A meter is one of the units of length.
OR
3) The meter is a unit of length.

Hope that helps.
Bye

三句都錯，
The unit of length is in metre.

The unit of length is meters

Answer is B or you can use "The metre is a unit of length"

The answer is the last one:

The unit of length is metres.

單位的長度是米。