Which thesis statement is strong because it demonstrates confidence?

a$.)$

I feel that the unsanitary living conditions of the urban poor in the early $20$th century contributed to the high infant mortality rate of the time.

b$.)$

The main reasons for the high infant mortality rate in the early $20$th century were government corruption and unsanitary housing.

c$.)$

It's possible that if city governments had better enforced existing housing and sanitation laws in the early $20$th century, the infant mortality rate would have been lower.

d$.)$

I believe that the infant mortality rate in the early $20$th century was negatively affected by overcrowding in cities and a lack of access to healthcare.