Which of the following statements is/are correct for the phylum Chordata?
$(a)$ In Cephalochordata,the notochord extends from head to tail region and is persistent throughout their life.
$(b)$ In Vertebrata,the notochord is present only during the embryonic period.
$(c)$ The central nervous system is dorsal and hollow.
$(d)$ Chordata is divided into $3$ subphyla: Hemichordata,Urochordata,and Cephalochordata.

• A
  $(d)$ and $(c)$
• B
  $(c)$ and $(a)$
• C
  $(a)$ and $(b)$
• D
  $(b)$ and $(c)$