I have a question about restrictive and non-restrictive relative clauses.
These are the original sentences:
The first plants to grow are pioneer plants. Pioneer plants can survive in harsh conditions.
Now if I am supposed to combine these sentences, which one is correct?
a) The first plants to grow are pioneer plants which can survive in harsh conditions.
b) The first plants to grow are pioneer plants, which can survive in harsh conditions.
I already know the difference between sentences, however, in a book called "Writing for IELTS", the second one is considered correct. But I, myself, think the opposite.