Each sentence below has three underlined parts ...

Answer: 
a

Each sentence below has three underlined parts having words or phrase marked A,B and C.Identify one of the underlined parts that must be changed to make the sentence correct.If the sentence needs no correction,the answer D is to be marked. (a)Hearing the strange noises, it (b)at once occurred to me that some one (c)had entered the house .(d)No Error