Which of the following statements are correct?
a) In maltose,$C-1$ of one $\alpha-D$-glucose is linked to $C-4$ of second $\alpha-D$-glucose.
b) Quaternary structure of protein represents spatial arrangement of two or more polypeptide chains with respect to each other.
c) Nucleotide units in a nucleic acid are joined to each other through a phosphodiester linkage.
d) Abnormally low level of thyroxine leads to hypothyroidism.

• A
  $a, b, c$
• B
  $a, b, c, d$
• C
  $b, c$
• D
  $a, c, d$