When Lomov comes to propose, he mentions that his Oxen Meadows border the Chubukov estate. Natalya immediately interrupts and claims the Meadows belong to her family, not Lomov. She firmly insists, "They're ours, ours, ours!" and threatens to send her mowers there that very day. This assertion of ownership angers Lomov, turning what should have been a marriage proposal into a heated quarrel over land. Natalya's stubborn refusal to accept Lomov's claim, and her aggressive tone, provoke him into shouting and exchanging insults.
Source: The Proposal, Chapter 9
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The examiner expects you to identify the trigger (mention of Oxen Meadows), Natalya's immediate contradicting claim, and her aggressive/stubborn stance as the three elements that provoke the quarrel. Note that the quarrel starts before Natalya even knows Lomov has come to propose — the irony is key to the play's humour and worth mentioning briefly. Avoid retelling the whole plot; focus on how Natalya provokes, not just that they quarrel.