Let \( R \) be a relation defined on the set \( \{1,2,3,4\} \times \{1,2,3,4\} \) by \( R = \{((a,b),(c,d)) : 2a + 3b = 3c + 4d\} \). Then the number of elements in \( R \) is:
\( 18 \)
\( 12 \)
\( 15 \)
\( 6 \)