Topic sentences for IELTS – how to write a high band score topic sentence

Not all paragraphs are about reasons.

If you're writing main body 1 for the question above, your paragraph is not about reasons; it's about areas of progress, or examples of areas of progress.  You could adapt the technique from above like this:

There are two main areas in which mankind has made progress.

Mankind has made great progress in two main areas, namely in medical science and in communications technology.

To summarise

Simple topic sentences

There are two main reasons why people change their jobs.

People change their jobs for various reasons.

People change their jobs for two main reasons.

Extended topic sentences

There are two main reasons why people change their jobs, namely through choice and necessity.

People change their jobs for reasons which are related to choice and necessity.

People change their jobs for reasons related to choice and necessity.

People tend to change their jobs because they have a choice or because they have to.

You could adapt these high band score topic sentences to many different main ideas.  Here are some examples:

Obesity is on the rise because people consume poor diets and do not do enough exercise.

There are two main reasons why obesity is on the rise, namely because of poor diets and a lack of exercise.

Online shopping is advantageous to consumers for two main reasons, namely because it is convenient and cheap.

There is one main reason why raising the price of petrol will reduce traffic congestion.

All the sentences shown above would be perfectly good as topic sentences for writing Task 2.  They do their job which is to tell the reader what the paragraph is about and, in most cases, what the supporting ideas are.