Type of compounds containing oxygen in which the first member has least molecular mass

first member of alcohol is ${\mathrm{CH}}_3\mathrm{OH}$ ; molecular weight is 32 

first member of ether is ${\mathrm{CH}}_3-\mathrm{O}-{\mathrm{CH}}_3$   molecular weight is 46

first member of Aldehyde $\mathrm{HCHO}$  molecular weight is 30

first member of ketone ${\mathrm{CH}}_3-\mathrm{CO}-{\mathrm{CH}}_3$molecular weight is 58

$\therefore$ The least molecular weight  is Formaldehyde (HCHO)