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1.The revolving currents of this molten iron at Earth's center act as a gigantic electromagnet.
act as 充当；扮演
That sofa also acts as a bed.
那沙发也可当床。
2.The last time was 780,000 years ago, so the next flip is well overdue.
overdue 迟到的；未兑的
The landlady is angry because the rent is overdue.
女房东生气了，因为房租过期未付。
