No, all the fruits and vegetables do not undergo browning. The cells of certain fruits like apples contain an enzyme called tyrosinase when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments. So, the fruits and vegetables which contain tyrosinase undergo browning.

No, rusting is not good for iron materials. Rust is formed when an iron surface comes into contact with water and oxygen. It results in the degradation of iron. Thus the metal will be weakened and become brittle and flaky.

Acid rain is caused by the chemical reaction when compounds like sulphur dioxide and nitrogen oxides are released into the air. The exhaust from cars and truck and the smoke released from industries emit enormous nitrogen oxides and sulphur dioxide into atmosphere. It forms the principal cause for acid rain.

Burning fossil fuels is a cause of global warming because it produces Co, and methane. These gases give the greenhouse effect. This increase the Earth's average temperature and results in the phenomenon called Global warming.

Yeast is a biocatalyst when yeast is added while making cakes
i) it triggers fermentation
ii) it releases carbondioxide which results in the expansion of dough.
iii) it is responsible for the soft porous texture of the cake
iv) It is the keyfactor behind the characteristic bakery flavour of a cake.