Which Is Correct? Once a Defendant Has Been Served with the Claim Form

Which is correct?
Once a defendant has been served with the claim form and particulars of claim …

A) he must file a defence or an admission within 28 days of the date of service upon him of the claim form and particulars of claim.
B) he must file either an acknowledgement of service or a defence or an admission within 14 days.
C) he must file either a defence or an admission within 14 days.
D) he may file only the acknowledgement of service within 14 days of service upon him of the claim form and particulars of claim and then he may have a further 28 days in which to file either his defence or admission.