Identify which are expressions and which are not.?

Question

It is estimated that the Earth is losing 4000 species of
plants and animals every year. If S represents the number of species living last
year, how many species are on Earth this year?

Answer 1

There were S species last year and now there are 4000 fewer species, so

This year we have S - 4000 species

Answer 2

If S is the number of species in 2006, then the number of species in 2007 is "S - 4000". This assuming of course that there are no other factors, like new species having been formed since then.

Answer 3

S - 4000.