Can I book a property with only Katha B?

I am planning to book a flat in Bangalore. But the builder is saying that it has only Katha B.
Is it safe for me to book this flat?
Can I apply for Katha A after taking possession of the flat? Or is it the builder’s responsibility to give Katha A while handling over?

What problems I may incur if i but a flat with only Katha B?

Please help.

thanks
Gopal