Need advice on answering "why" question, thanks again, teachers!

[Help Wanted]

Dear teachers again,

Sorry for wanting more advice.
Q1
I was wondering if the following way of answering a "why" question is correct or not.
Why do....?
This is because.... ( this is definitely acceptable, as far as I know)
What about.... ?
That is because....( please advice, teachers!)

Q2
Can we write:
He could not win the race.
OR
He could not win in the race.
(Which one is correct or both are..)

Q3
The rat made the cat lost (lose???) in the race.

Many thanks for advising!

[AintFoolin]

not positive what you're asking, but that won't stop me from taking a stab at it ;)

i don't know if it's grammatically correct (i suspect not), but in spoken English, your response would usually just be "Because . . ."

Why is the sky blue?
Because water molecules in the atmosphere diffract blue light better.

if you aren't comfortable with that, just leave off any intro and answer the question straight up

Why is the sky blue?
Water molecules in the atmosphere diffract blue light better.

[Help Wanted]

Thank you for your advice again, teacher AintFoolin!

[Casiopea]

Q1
I was wondering if the following way of answering a "why" question is correct or not.
Why do....?
This is because.... ( this is definitely acceptable, as far as I know)
What about.... ?
That is because....( please advice, teachers!)

The words in brackets (...) can be left out:

Q: Why do you eat seaweed?
A: (I eat seaweed beacuse) it's healthy.

Q: What about tofu?
A: (I eat tofu because) it's healthy.