They danced as if they__________practicing for weeks.

Complete Sentence: They danced as if they had been practicing for weeks.

→ As if/as though এর পূর্বে Past indefinite tense থাকলে as if/as though এর পরে Past perfect tense হয়।
→ যেহেতু প্রশ্নে প্রদত্ত বাক্যটিতে as if এর পূর্বে Past indefinite tense রয়েছে, তাই নিয়মানুযায়ী সঠিক উত্তর হবে - had been.

অন্যদিকে,
→ As if/as though এর পূর্বে Present indefinite tense থাকলে as if/as though এর পরে Past indefinite/perfect tense হয় এবং be verb থাকলে সবসময় were হয়।
যেমন: He spoke as if he were mad.

Example Sentence:
1. The child spoke as if he had been reciting a memorized poem.
2. The musician played the instrument as if he had been born with a natural talent.

Source: A Passage to the English Language, S. M. Zakir Hussain.