Which of the following is an example of a simile?

a. the sister arts enjoy the use of a plastic and ductile material, like the modeller's clay; literature alone is condemned to work in mosaic with finite and quite rigid words.
b. it is with blocks of just such arbitrary size and figure that the literary architect is condemned to design the palace of his art.
c. but though this form of merit is without doubt the most sensible and seizing, it is far from being equally present in all writers.
d. the effect of words in shakespeare, their singular justice, significance, and poetic charm, is different, indeed, from the effect of words in addison or fielding.

i'm stuck between a and d.