Pick out the most effective word/phrases from the given alternatives marked (a), (b), (c), (d) and (e) given below each sentence to fill in the blank to make the sentence meaningful and grammatically correct.

It is difficult to speak a language fluently unless ______ regularly.

A. it is in practice

B. it is by practising

C. it will be practised

D. it is practised

Please do not use chat terms. Example: avoid using "grt" instead of "great".