Which one of the following is correctly described as a cell part?

• A
  Centrioles $\rightarrow$ Sites for active $RNA$ synthesis.
• B
  Lysosomes $\rightarrow$ Optimally active at a $pH$ of $8.5$ or higher.
• C
  Thylakoids $\rightarrow$ Flattened membranous sacs forming the grana in chloroplasts.
• D
  Ribosomes $\rightarrow$ Those on chloroplasts are larger $(80S)$ while those in the cytoplasm are smaller $(70S)$.