Big Ben is the UK’s most iconic symbol. Since 2017, the Elizabeth Tower and the bell inside have been (1)_________ renovation. The structure is 160 years old and (2)___________ regular maintenance. (3)__________ many years, people had been painting the hands and numbers (4)________ the clock black, but last Thursday, the workers (5)___________ the clock’s original colour, Prussian Blue!
Select the most appropriate option for blank No. 4
(a) to
(b) of
(c) at
(d) for