What is the formula for theme

Imagination! who can sing thy force? or who describe the swiftness of thy course? soaring through air to find the bright abode, th' empyreal palace of the thund'ring mode, we on thy pinions can surpass the wind, and leave the rolling universe behind: from star to star the mental optics rove, measure the skies, and range the realms above. there in one view we grasp the mighty whole, or with new worlds amaze th' unbounded soul. what does the speaker compare imagination to in the poem? a. a soaring bird b. the sky c. a cloud d. a sailing ship