I think JY messed up explaining why option A for question 20 doesn't work

the contrapositive of /PS → GEF is /GEF→PS; this is the form in which proper sequencing is a necessary condition for gene editing not being flawed. There are two ways to look at it: not having proper sequencing as the subset, and gene editing being flawed as the superset, OR gene editing not being flawed as the subset, and proper sequencing as the superset

the contrapositive of unfamiliar is "not unfamiliar", not "familiar". Just like how the contrapositive of ignorant is not ignorant, as opposed to "knowledgable" or "smart"

For those confused, this is how I understand it: If new restaurants open, that is a sufficient condition for living standards to improve (we know this happens for sure). The improvement of living standards is a necessary condition of new restaurants opening; new restaurants cannot open without improving living standards -but, improved living standards is not sufficient for new restaurants having opened, because living standards could improve for other reasons.

i wouldn't stress it- a lot of this stuff, you likely understand intuitively, and now that its being explicated, you are just overthinking it

For question 12.1 – why is there only one premise, in comparison to questions 10.1 and 10.2 where there are two premises?