Why Was the Transition to Settled Agriculture Slower in Western

Why was the transition to settled agriculture slower in western Afro-Eurasia than in the rest of the landmass?

A) The techniques adopted by Southwest Asians never spread to northern Europe.
B) There were no river valleys in Europe to serve as the basis of agriculture.
C) Inventions such as the plow or metal working were not known in Europe.
D) The large population of hunter-gatherers resisted moving to settled agriculture.