Answer:
A: There is a shift in topic.
B: There is a stanza break.
Step-by-step explanation:
Sonnet 18 is a Shakespearean sonnet which consists of 14 lines of iambic pentameter (three quatrains which are followed by a couplet). The rhyme scheme is also characteristic of Shakespearean sonnet: ABAB CDCD EFEF GG.
In the sonnet, the speaker compares a young man to a sunny day, noting that the qualities of summer weather will eventually change.
In the third quatrain, the speaker interrupts this comparison, while the use of word 'but' marks the shift in topic. The speaker then introduces a new idea and claims that the man will live forever in this sonnet.