Meaning of "physical supply chain"

[Sasooo]

Business English Lesson, Advanced Level

ESL/EFL Test #555 "The Physical Supply Chain", question 1

The reduction of uncertainty in the physical supply chain requires cooperation between trading partners; this uncertainty is reduced ......... but requires adequate investment from both of the partners.

(a) incidentally
(b) incrementally
(c) indelibly
(d) irrevocably

Business English Lesson, Advanced Level

ESL/EFL Test #555 "The Physical Supply Chain", answer 1

The reduction of uncertainty in the physical supply chain requires cooperation between trading partners; this uncertainty is reduced incrementally but requires adequate investment from both of the partners.

Correct answer: (b) incrementally
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I think this test is some difficuilt to me please explain (physical supply chain)

[Mister Micawber]

physical supply chain = any sequence of processes involved in the production and distribution of a commodity; the network of retailers, distributors, transporters, storage facilities and suppliers that participate in the sale, delivery and production of a particular product.
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Native English teacher at Mister Micawber's