Then , the grand finale rumble ( or bottom burp , as Ian refers to it ) , went echoing first through my chamber , then , seconds later , through hers . I put my hand to my mouth in a dramatic gesture of embarrassment . Isla did the same . I giggled .

Pick from:
(a). It is something that is or sounds like flatulence .;
(b). It is the sound a person makes when they are embarrassed and giggle .;
(c). A bottom burp is something that goes through pipes quickly .;
(d). It is a rumble of a car or some other large chambered vehicle .;
Q: What might Ian mean when he calls something a " bottom burp . "
(a).