Endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins
"Across the street" is the prepositional phrase in this sentence. Best friend lives where? Best friend lives across the street. The prepositional phrase in this sentence indicates C) where the best friend lives.
