Option (B) was diagrammed this way because of its structure.
"Jenny will have lots of balloons at her birthday party."
Hypothetically, we could either rewrite this as "if it is Jenny's birthday party, then there will be lots of balloons" or "if there are lots of balloons, then it is Jenny's birthday party."
The first way is correct for two reasons:
(1) The focus in this statement is on Jenny and her party, not on the presence of balloons. Therefore, it makes more sense grammatically to diagram it this way.
(2) It does not make sense that the presence of lots of balloons is sufficient for it to be Jenny's birthday party. There are plenty of reasons why there may be lots of balloons e.g. someone else's birthday party, a retirement party, a baby shower, etc. It makes more sense that Jenny's birthday party is sufficient for the presence of balloons.
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