A company makes four kinds of products. Each product has a size code, a weight code, and a shape code. The following table shows these codes:

Size Code

Weight Code

Shape Code

$1$

$42$

$27$

$42$

$2$

$27$

$38$

$13$

$3$

$13$

$12$

$27$

$4$

$42$

$38$

$38$

Find which of the three codes is a primary key.