What is one of the main threats to bridges caused by humans?

Read the excerpt below from the poem "exile" by julia alvarez and answer the question that follows. i let myself lie back in deep waters, my arms out like jesus' on his cross, and instead of sinking down like i'd always done, magically, that night, i could stay up, floating out past the driveway, past the gates, in the black ford, papi grim at the wheel, winding through the back roads, stroke by difficult stroke, out on the highway, heading toward the coast. which literary device is used in the comparison between the speaker's experience of fleeing the country and floating in the ocean? i let myself lie back in deep waters, my arms out like jesus' on his cross, and instead of sinking down like i'd always done, magically, that night, i could stay up, extended metaphor symbolism simile theme