1-2-3-4-5-6,a; otherwise there is no one-to-one correspondence or deductive differentiation. Use your visual mind to 'see' the action: 1 some wall(s) of some building(s); (that's all you can deduce) 2 some wall(s) of some building(s); (there is no extra information) .. 6 some wall(s) of some building(s); (there is no more information) Adding 'the' is not a magic participle, it provides no additional 'action' on the subject.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.