With Respect to the DNA Methylation Mechanism in Mammalian Cells

With respect to the DNA methylation mechanism in mammalian cells, which of the following statements, if any, is true?
a) The principal role of the DNMT1 DNA methyltransferase is in de novo methylation.
b) The DNMT3A and DNMT3B DNA methyltransferases require hemi-methylated DNA as a substrate and are responsible for methylating nascent DNA strands that are complementary to methylated parental DNA strands
c) Active DNA demethylation means removal of methyl groups from a hemi-methylated DNA double helix.
d) DNA methylation is not essential in mammalian development.