Difference between to extend and to expand

[St?phanie]

Test No. incompl/advan-16 "At the Shops", question 4

It started off as a small street corner shop over 30 years ago but it has now ......... into a massive department store.

(a) expanded
(b) extended
(c) explored
(d) enlarged

Test No. incompl/advan-16 "At the Shops", answer 4

It started off as a small street corner shop over 30 years ago but it has now expanded into a massive department store.

Correct answer: (a) expanded

Your answer was: incorrect
It started off as a small street corner shop over 30 years ago but it has now extended into a massive department store.
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What is the difference between to extend and to expand? and when do we have to use them?

[Mister Micawber]

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Extend comes from the Latin tendere, to stretch; expand comes from Latin pandere, to spread.

In both physical and metaphorical use, extend tends to be used for enlargement along a single axis or in a single direction (my boundary extends from A to B), where expand tends to mean spreading out in many directions (gas expands to fill a vacuum). The latter is the sense meant in this question.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber