Spanish language Q&A Forum

There was a question in one of the quizzes I took and it led me to this grammar topic (questions with qué):

Which flowers do you prefer?  

¿ _____ flores prefieres ? 

I put cuales in the blank, but was marked wrong, it said the answer was qué. 

I went to the lesson on cual/cuales (interrogative pronouns) and there was a sample sentence referring to flowers, using cuales prefieres.  

In the English translation to the quiz it uses which and I assumed that a select few flowers were being referred to, thus the answer should be cuales. It is a bit ambiguous. 

Happy birthday to you.  Why is ti used and not te?   I can’t find a lesson on the use of ti instead of te. For instance para ti

In the introduction to this lesson, you say the meaning is similar to using "haber plus infinitive". I think you mean haber plus past participle i.e. the perfect tense

"Hurry, Run!"  Can I interpret "Hurry" as Command, too? If so, none of the options is appropriate. 

Aunque tengo una casa muy lujosa, prefiero la vuestra. I'm confused. Where does 'vuestro' come from? What happened to 'tuyo' and 'suyo'?

Question:

Queda muy ________ arroz, no podemos hacer la paella.

Answer: poca

I understand the explanation of when to use poco/poca/pocas/pocos but I don't understand why in this circumstance it is feminine, because I thought arroz is masculine. Is 'arroz' actually feminine, or is it because 'la paella' is feminine?