Difference between lengthy and long

[Cruella]

Test No. errors/advan-7 "At the Meeting", question 5

It was a difficult topic to discuss but after lengthening discussions it was finally agreed that the whole business should be looked at by a much smaller group.

(a) topic
(b) lengthening
(c) agreed
(d) should be

Test No. errors/advan-7 "At the Meeting", answer 5

It was a difficult topic to discuss but after lengthy discussions it was finally agreed that the whole business should be looked at by a much smaller group.

Correct entry: lengthy
The error was: (b) lengthening

You have found the error but your entry is incorrect.
It was a difficult topic to discuss but after long discussions it was finally agreed that the whole business should be looked at by a much smaller group.
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why not "long"

Cruella

[Torsten]

You need lengthy here because this adjective describes a meeting or a discussion which is tediously long and often exhausting.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary