Which of the following is an example of an abab design? half of the subjects receive one treatment and the other half are not treated. all subjects received one of two treatments. a subject is observed and treated. a subject is observed both before and after two exposures to the treatment.

Option D, A subject is observed both before and after two exposures to the treatment.

Explanation:

In A-B-A-B design the alphabets have following meaning

a)First A – It is the baseline value or the value measured before treatment

b)First B – It is the treatment measurement

c)Second A  - It is the withdrawal of treatment  

d)Second B – It is the reintroduction of treatment  

The second A is known as the reversal phase in which the intervention is withdrawn to see if the target behavior returns to the baseline behaviour

Thus , there are two measurements involved one before the intervention and second after the intervention

Option D is correct

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.

easy

explanation: