Meaning of ひとつは in Japanese

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Definition of ひとつは

ひとつは(hitotsuha) 【一つは ·一は】

The following is thought to be one of the reasons stress damages the stomach's mucous membrane.

大都市だいとしの魅力みりょくのひとつは、その建物たてものの建築けんちく様式ようしきに見られる多様性たようせいにある。

Part of the charm of a big city lies in the variety of styles that can be seen in the architecture of its buildings.