WC change Select Option text while variable product

function zakra_product_add_to_cart_text( $string ) {
    global $product;

    if ( $product->is_type( 'variable' ) ) {
        $string = $product->is_purchasable() ? esc_html__( 'Buy Now', 'zakra' ) : esc_html__( 'Read more', 'zakra' );
    }

    return $string;
}
add_filter( 'woocommerce_product_add_to_cart_text', 'zakra_product_add_to_cart_text', 10 );