"Ce" à place de "Elles" à la première phrase

Question

Pourquoi c'est permis pour "ce" être à place de "Elles" à la première phrase?

Accepted Answer

Hi Dan,
You can either say 
Elles sont jumelles 
or
Ce sont des jumelles
for
They are twins
It is the same as 
Elle est bonne nageuse = C'est une bonne nageuse ( She is a good swimmer)
It is the old favourite,  take a look at the Q&A attached to this lesson for lots of examples 
"C'est" vs "Il/Elle est" to say it is/she is/he is in French
Hope this helps!

Answer

Both options are acceptable, they just place the emphasis on two different aspects:

Elles sont jumelles -- Ce sont des jumelles -- They are twins.

Using elles is what you'd think of first, coming from English. Here you think of the two girls individually and the proper pronoun to use is elles. However, in French you can also think of them as an impersonal group and say "this are twins". I know, that doesn't work in English, but it does in French. Hence the use of ce as another possibility.

Answer

Thankbyou. It is clearer now.