In Relation to the Movement of Assimilates in Plants

In relation to the movement of assimilates in plants

A) sources are always sites of photosynthesis.
B) sinks may import assimilates from storage reserves or from sites of photosynthesis.
C) sinks tend to be supplied equally by neighbouring and distant sources.
D) a structure cannot switch between being a source or a sink.
E) sources are always roots.