The simile that likens his shoulders to a full sail shows the force that must be exerted to plough the field. A "globed" sail would be rounded because of the powerful winds blowing against it. Just as the wind's, resistance is transformed into something useful by moving the boat forwards; his father's exertion transforms the land into fields that grow crops of food. While ploughing the land in this way would certainly require knowledge and skill, the simile does not refer to these qualities.
2. </span><span>D. "Mapping the furrow exactly"
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Reference to the father's expertise is indicated by his "mapping the furrow" and doing so "exactly."
