The adverb ‘swiftly’ modifies the ‘verb’ lowering, hence it is more suitable. To have parallelism in the construction, ‘lowering interest rates’ should be followed by ‘giving fiscal stimulus.’ ‘Be covered’ is correct in sentence B for it has to be passive. In C as ‘for selling’ is used in the later part of the sentence hence ‘for buying’ should be preferred in the previous part. In sentence D we are talking about something related to the present ‘now’ so the present perfect tense ‘have started’ should be preferred.