Can't catch radio button in widget

I want to create a custom widget with 3 simple radio buttons, so i made class with WidgetName, widget(), update(), form(). I will show update(), and form(), because i think they are important :

function update( $new_instance, $old_instance ) {

		$instance = $old_instance;

		$instance['default-image'] = $new_instance['default-image'];
		$instance['post-image'] = $new_instance['post-image'];
		$instance['link-image'] = $new_instance['link-image'];

		return $instance;
	}

function form( $instance ) {

		$defaults = array('default-image' => true, 'post-image' => false, 'link-image' => false );

		$instance = wp_parse_args( (array) $instance, $defaults ); ?>

		<p>
			<input name="option" type="radio" <?php checked( $instance['default-image']); ?> id="<?php echo $this->get_field_id( 'default-image' ); ?>"/>
			<label for="<?php echo $this->get_field_id( 'default-image' ); ?>"><?php _e('Default image'); ?></label>
		</p>
		<p>
			<input name="option" type="radio" <?php checked( $instance['post-image']); ?> id="<?php echo $this->get_field_id( 'post-image' ); ?>" />
			<label for="<?php echo $this->get_field_id( 'post-image' ); ?>"><?php _e('First post image'); ?></label>
		</p>
		<p>
			<input name="option" type="radio" <?php checked( $instance['link-image']); ?> id="<?php echo $this->get_field_id( 'link-image' ); ?>" />
			<label for="<?php echo $this->get_field_id( 'link-image' ); ?>"><?php _e('Image from link', 'example'); ?></label>
		</p>

	<?php
	}

but in this scenario, when i refresh admin page on click on “Save” button, the current selection disappears. I’m new in wordpress and php, so sorry for “trivial” question. I will appreciate every help.