In terms of pronominal reference,...

A

“…that…” (line 2) refers to “…blood…” (line 1).

B

“…which…” (line 11) refers to “…supplies…” (line 10).

C

“where…” (line 16) refers to “…networks” (line 15).

D

“…where…” (line 31) refers to “…routes…” (line 31).

E

“This…” (line 37) refers to “…behavior.” (line 37).