Since John Locke acknowledged authorship of his political works only in a codicil to his will, the period during which the Second Treatise on Government was written has been established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, what is much more difficult to determine are the personal reason Locke wrote the Treatise, the changes he might have made to his first version, and the extent to which the published version coheres with Locke's intentions.

a. has been established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, what is much more difficult to determine are

b. has been established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, what is much more difficult to determine is

c. have been established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, what is much more difficult to determine is

d. have been established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, what is much more difficult to determine are

e. are established through a close analysis of Locke's reported activities near the time of publication; however, because no original manuscript has been found, that which is much more difficult to determine is

This one is throwing me for a loop. What does the crowd think?

Thanks guys, OA was B. I picked A and was having difficulty wrapping my head around "is A, B, and C."

Ryan