Consider the following regression equation: y $=$ B$1+$ B$2$x$1+$ B$2$x$2+$ u$.$ What does Bi imply?a$.$ B$1$ measures the ceteris paribus effect of X on X$2.$ B$1$ measures the ceteris paribus effect of y on X$1$ c$.$ B$1$ measures the ceteris paribus effect of xion y$.$d$.$ B$1$ measures the ceteris paribus effect of X$1$ on u$.$