The answer is A.
The subject of the sentence (the person or thing doing the action) has to agree in number with the verb (the word representing the action).
The author has the batter answer ir describe the situation in matter-of-fact sentences
verb
make or become hard or solid.
"the magma slowly solidifies and forms crystals"
Similar:
harden
go hard
set
freeze
ice over/up
gel
thicken
stiffen
congeal
clot
coagulate
curdle
cake
dry
bake
consolidate
ossify
fossilize
petrify
gelatinize
Opposite:
liquefy
melt
thaw
gasify
make stronger; reinforce.
<span>A small, glittering object not far away caught Rainsford's eye and he picked it up. It was an empty cartridge.</span>
