Read the passage.

(1) edgar allan poe wrote the short story “the tell-tale heart.” (2) poe wrote the story in 1843. (3) in the story, a man is haunted by an old man’s eye. (4) he thinks it looks like the eye of a vulture. (5) we soon learn that the haunted man has a mysterious disease. (6) the disease has made the man insane. (7) he does not know it. (8) for seven nights, he sneaks into the old man’s room. (9) he watches the sleeping man. (10) i will not tell you what happens next because i do not want to spoil the story for you.
which is the most effective way to combine sentences (6) and (7)?

a. the disease has made the man insane he does not know it.
b. making the man insane, the disease is not known by him.
c. the disease has made the man insane, but he does not know it.
d. not known by him, the disease has made the man insane.