External supply risk of agricultural products trade along the Belt and Road under the background of COVID-19.

From the probability distribution diagram of degree ( Figure 2 ), the exponential probability distribution of degree, in-degree and out-degree of agricultural products trade network along the route in 2019 and 2021 shows a decreasing trend: the number of nodes with higher degree is rare, unlike the number of nodes with lower degree is large, which is an important feature of scale-free distribution network ( 34 ).

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