A Cloze Test Question - London

London, which is the largest city in Europe, is home to about seven million people. (28) ---- by the Romans in the 1st century A.D. as an administrative centre and trading port, the capital is now the main residence of the British monarchs, and the centre of government. (29) ---- possessing many museums and art galleries, London has many other attractions, which make it an exciting city, (30) ---- a wide variety of entertainment. Various cultural developments that have taken place recently (31) ---- to the city’s range of attractions. However, there are many other (32) ---- towns to explore, such as the historic centres of York and Bath.


31. A) had added B) were adding C) have added D) will add E) should have added


From these questions, the question whose answer I really wonder is the 31th question. This cloze test is taken from an official foreign language exam made all over Turkey. The offical answer is C, but why not D? Is it possible that D also works?

Thanks in advance