I have been asked to verify the effect of water and sanitation on the mortality of children under the age of 5 and quantify whether providing water services or sanitation services has a larger effect on child mortality.

where
mortality rate of children under the age of 5 (per 1000 live births) - INFMORT
GDP per Capita - GDPPC
% with access to basic water services - WATER
% with access to basic sanitation services - SANIT

My starting model was INFMORT = b0 + b1 (GDPPC) + b2 (WATER) + b3 (SANIT)

I was wondering whether anyone had any suggestions as to the correct functional form to use as all the variations I have tried have delivered unexpected results.