Why is «مَسَاجِدَ» genitive in «مَرَرْتُ بِمَسَاجِدَ»?

Noun Case Classes: Type 3 — Diptote (Ghayr Munṣarif)

• Because it is a sound feminine plural
• Because it follows the preposition bi-
• Because it is a subject
• Because it has the article al-