請幫mang

1. The colour of a leave is determined by the pigment it possesses. Usually, it contains four main types of pigments - chlorophyll a, chlorophyll b, carotene and xanthophyll. The former two are green in colour which is an ordinary colour for a leaf whereas the latter two are orange and yellowish brown respectively. The colour of the leaf is green because the colour of chlorophyll a and b (green) masks that of carotene and xanthophyll. During autumn, due to environmental factors such as humidity or amount of water in soil, etc, the colour of the leaves changes to yellow because the rate of chlorophyll formation is slower than that of chlorophyll decomposition. Therefore, fewer amount of chlorophyll to mask the colour of carotene and xanthophyll which are orange and yellowish-brown respectively, the dominant colour of the leaf becomes orange or yellow.

6. There are two types of twins - identical and fraternal twins.
The genetic composition of the former one is nearly 100 % the same whereas that of the latter one is not the same.
This is because identical twins are formed by the splitting of a mitotically dividing fertilised egg into 2 parts, each part develops into a baby. As these 2 parts come from the same origin, they have nearly identical genetic composition and hence nearly identical phenotypes. This type of twins involves 1 egg being fertilised by 1 sperm only. However, as for the fraternal twins, it involves 2 eggs and 2 sperms. Each egg is fertilised by a sperm. Then, this gives rise to two genetically different babies.
Some pairs of twins do not look alike because they are fraternal twins in lieu of an identical one.