Phosphorylation of enzymes:. a. always increases their activity.
b. generally occurs on Ser, Thr, and/or Tyr side chains and to a lesser extent on the His side chain.
c. is irreversible.
d. is one of only five known covalent forms of regulation.

generally occurs on Ser, Thr, and/or Tyr side chains and to a lesser extent on the His side chain

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current

level 4 to level 2 : ))