A student investigates the polarization of microwaves from two different sources, $M_1$ and $M_2$, using the experimental setup shown below.

A metal grille is placed between source $M_1$ and the receiver. When the grille is rotated through a small angle from its initial position, the detected signal strength from $M_1$ decreases.

When the same grille is placed between source $M_2$ and the receiver and rotated through any angle, the detected signal strength remains constant.

Which row in the table correctly identifies the polarization state of the microwaves from $M_1$ and $M_2$?