a)  Incorrect.  CH₄ is a tetrahedral molecule.  Each of the bonds is polarized toward the carbon atom.  Adding together any two bond dipoles
            gives a subtotal sum halfway between them directed toward the central carbon atom.  When the two subtotal sums are added, they will cancel
            each other out (identical forces directed head-to-head) and the molecule is nonpolar.  CH₂O is triangular planar.  Adding together the two
            C-H bonds gives a subtotal sum halfway between them directed toward the central carbon atom.  When this subtotal sum is added to the
            bond dipole for C-O double bond, they don't cancel each other out (in fact, they reinforce each other) thus the molecule is polar. CH₃Cl is
            a tetrahedral molecule.  Each of the three C-H bonds is polarized toward the carbon atom while the C-Cl bond is polarized toward the
            chlorine atom.  Adding together two C-H bond dipoles gives a subtotal sum halfway between them directed toward the central carbon
            atom.  Adding together the third C-H bond and the C-Cl bond dipoles gives a subtotal sum which won't cancel out the other subtotal
            sum (since the combination is different) thus the molecule is polar.