Ah!, the wonderful 'New Concept English' - used extensively throughout China and full of grammatical errors and complete 'f*ck-up's'

Choose the CORRECT answer to complete the sentence -

'Next Saturday we.....................a party'

a) are going to have b) will have c) might have d) are having

The 'correct' answer is 'A' (apparently)

Any of you fancy the job of explaining to students why the other 3 are 'wrong'? .....What about you, Doc?