Machines were made to be man's servants. Yet, man has grown so (1)______ on them that they are in a fair way to become his (2)______. Already men spend most of their lives looking after and waiting (3)______ machines. Machines are very stern bosses. They must be fed with coal and (4)______ petrol to drink and oil to wash with and must be kept at an (5)______ temperature.
Select the most appropriate option for blank no. 4