4. Determine whether the relation \( R \) on the set of all people is reflexive, symmetric, antisymmetric, and/or transitive, where \( (a, b) \in R \) if and only if
a) \( a \) is taller than \( b \).
b) \( a \) and \( b \) were born on the same day.
c) \( a \) has the same first name as \( b \).
d) \( a \) and \( b \) have a common grandparent.