Why is it more difficult to breathe at high altitudes rather than low

Because the air gets thinner. The air at higher altitudes is more uncontaminated oxygen, cleaner air, whereas at low altitudes, air is more contaminated by smog and pollution from factories, power plants, etc.

As you travel higher, the air begins to thin out which makes it harder for you to breathe. There is less oxygen as you travel higher.

The correct answer is "hydrogen".

Hbr dissociates to form h+ and br-ca(oh)2 dissociates to form ca2+ and 2oh-the h+ and oh- react to form waterassuming the reactants are dissolved in water, the net reaction is h+ + oh- —> h2oif you’re asked to show all components of the reaction, the complete reaction is 2hbr + ca(oh)2 —> cabr2 + 2h2o