Select the ways in which the amount of insolation reaching earths surface varies spatially.
a. in general, lower amounts of insolation are received at higher latitudes
b. variation over the oceans is greater than over the continents
c. regions just north and south of the equator receive more insolation than at the equator
d. the highest values of received insolation are in subtropical deserts due, in part, to low cloud cover