Which of these fits gap 21?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
Sneezes, coughs, watery eyes, running noses and sore throats, are common _____ (21) _____ that mark a person infected with the common cold. The common cold is a viral infection that seems _____ (22) _____. The reason is that antibiotics and ordinary medicine cannot treat a viral infection. The _______ (23) _______ reason is that the conditions, under which we say that somebody has caught a cold, can be _______ (24) ______ about by a range of viruses that list more than a hundred.