A radioactive nuclide that is used for geological dating has an atomic number of 19 and mass number 40. Which is the symbol of this nuclide?
A
Superscript 19 subscript 21 upper K.
B
Superscript 21 subscript 19 upper K.
C
Superscript 40 subscript 19 upper K.
D
Superscript 19 subscript 40 upper K.