Cell wall of diatom is deposited with silica. After the death of diatoms, cell wall is resistant to decomposition, piles up at the floor of water body and forms diatomaceous earth.

Hence, the correct option is (B).

A known mass of the organic compound is heated in the presence of pure oxygen. The carbon dioxide and water formed are collected and weighed. The percentages of carbon and hydrogen in the compound are calculated from the masses of carbon dioxide and water. The estimation of carbon and hydrogen in an organic compound is based on their conversion to CO, and H₂O respectively. The percentage of carbon and hydrogen is calculated from the masses of CO₂ and H₂O.

The apparatus consists of a long glass tube. This is called a combustion tube. To one end of this tube, a U-tube containing anhydrous calcium chloride and a bottle containing concentrated potassium hydroxide solution (caustic potash) are attached in series. These, in turn, are connected to a guard tube containing anhydrous calcium chloride.

Hence, the correct option is (C).

Primary pollutants include ammonia, sulfur dioxide, nitrogen dioxide and carbon monoxide. Secondary pollutants include ground-level ozone, acid rain and nutrient enrichment compounds.

Hence, the correct option is (C).

In vitro synthesis of DNA was achieved with the enzyme DNA polymerase which was isolated from lysates of E. coli by Arthur Kornberg in 1956. This enzyme synthesizes the segments of DNA complimentary to the parent strand. 

Hence, the correct option is (D).

Two identical point charges Q are kept at a distance r from each other. A third point charge is placed on the line joining these two charges such that all the three charges are in equilibrium. i.e., 

The blastocyst is a hollow structure that contains a cluster of cells from which embryo arises. The outer layer provides placenta and other essential tissues for fetal development while the inner cells give rise to the tissues of the body. This outer layer is known as trophoblast which provides nutrients to the embryo. Embryoblast is cells which are situated within the trophoblast and result in the production of an embryo.

Hence, the correct option is (B).

Growth in plants occurs as the stems and roots length. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in the length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem.

Hence, the correct option is (A).