Haemophilus ducreyi is a fastidious gram-negative coccobacillus bacteria, which causes sexually transmitted disease chancroid, a major cause of genital ulceration in developing countries, characterized by painful sores on genitalia. 

Tuberculosis, mumps and whooping cough are contagious types of diseases. The diseases can spread through droplets or aerosols which are formed during coughing or sneezing. These aerosols contain bacterium. The bacterium may enter or person's body who is healthy and cause infection. 

• The rapid increase in population over a relatively short period of time is called population explosion. 
• Reasons for population explosion are increased health facilities, decrease in MMR (maternal mortality rate), and IMR (infant mortality rate), and an increase in number of people reaching reproductive age.
• So, correct answer is option A.

The gestation period of pregnancy is divided into three trimesters. The second trimester starts from 13${}^{th}$ week to 27${}^{th}$ week of pregnancy. This is also termed as golden period of gestation. Significant changes takes place in development of baby. The organs develop in foetus and foetus increases in size.