Which compound in each of the following pairs would you expect to have the more polar carbonmetal

Question:

Which compound in each of the following pairs would you expect to have the more polar carbon–metal bond? Compare the models on Learning By Modeling with respect to the charge on the carbon bonded to the metal.
(a) CH3CH2Li or (CH3CH2)3Al
(c) CH3CH2MgBr or HC≡CMgBr
(b) (CH3)2Zn or (CH3)2Mg