在高等数学中，函数f(x)在点x₀处的导数定义为：当自变量x的变化量Δx趋于零时，函数值的增量Δy与Δx之比的_。
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Correct answer
A. 极限
Principle or equation
导数的定义是函数增量与自变量增量之比的极限。
Why this answer is correct
导数的定义：f'(x₀) = lim_{Δx→0} [f(x₀+Δx)-f(x₀)]/Δx，即增量之比的极限。因此空格处应填“极限”。
Example
f(x)=x²，则f'(2)=lim_{Δx→0} [(2+Δx)²-4]/Δx = lim_{Δx→0} (4Δx+Δx²)/Δx = 4。
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