Answer:
a. Russia relied on serfs and the serfdom system too much
Step-by-step explanation:
Why it was difficult for early tsars to free the serfs is because "Russia relied on serfs and the serfdom system too much."
This is evident in the fact that Serfs worked throughout the year for their masters, mostly Noblemen of Russia, with only a week of freedom. The serfs worked on the agricultural fields, but some were also used in domestic activities at home.