Which of the following statement(s) is/are correct?

• A

  Among the following elements: K, Mn, Cd, Cs, Fe, Cu, Pb, Os, Y. The number of transition element is 4.

• B

  All the lanthanides and actinides belong to III B or the 3rd group in the Periodic table.

• C

  The inner transition elements belong to the f' block of the periodic table and are shown separately at the bottom of the periodic table.

• D

  The d-block elements have variable valency.