The passage implies that the telegraph cable was built mainly ______.

A．dozens of people have rung him but haven't given him a job
B．he has applied for jobs but nobody ever replies to his letters
C．he has not seen any other jobs advertised
D．only four times have employers asked him to come and see them

A．to make some sound experiments in the oceans
B．to collect samples of sea plants and animals
C．to estimate the length of cable that was to be made
D．to measure the depths of two oceans