阅读下列短文，从每题所给的四个选项(A、B、C和D)中选出最佳选项。
Medical drugs sometimes cause more damage than they cure. One solution to this problem is to put the drugs inside a capsule，protecting them from the body-and the body from them－until they can be released at just the right spot. There are lots of ways to trigger(引发)this release，including changing temperature，acidity，and so on. But triggers can come with their own risks－burns，for example. Now，researchers in California have designed what could be a harmless trigger to date：shining near-infrared light(NIR，近红外线)on the drug in the capsule.
The idea of using light to liberate the drug in the capsule isn't new. Researchers around the globe have developed polymers(聚合物)and other materials that begin to break down when they absorb either ultraviolet(UV，紫外线)or visible light. But tissues also readily absorb UV and visible light，which means the drug release can be triggered only near the skin，where the light can reach the capsule. NIR light largely passes through tissues，so researchers have tried to use it as a trigger. But few compounds(化合物)absorb NIR well and go through chemical changes.
That changed last year when Adah Almutairi，a chemist at the University of Califomia，San Diego，reported that she and her colleagues had designed a polymer that breaks down when it absorbs NIR light. Their polymer used a commercially available NIRabsorbing group called onitrobenzyl(ONB)．When they catch the light，ONB groups fall off the polymer，leading to its breakdown. But ONB is only a so-so NIR absorber，and it could be poisonous to cells when it separates from the polymer.
So Almutairi and her colleagues reported creating a new material for capsules that's even better. This one consists of a long chain of compounds called cresol groups linked in a polymer. Cresol contains reactive(易反应的)components that make it highly unstable in its polymeric form，a feature Almutairi and her colleagues use to their advantage. After polymerizing the cresols，they cap each reactive component with a light-absorbing compound called Bhc. When the Bhcs absorb NIR light，the reactive groups are exposed and break the long polymer into two short chains. Shining additional light continues this breakdown，potentially releasing any drugs in the capsule. What's more，Almutairi says，Bhc is 10 times better at absorbing NIR than is ONB and is not poisonous to cells.
According to the passage，which of the following could be the best trigger?

A．Temperature change.

B．NIR light.

C．Acidity change.

D．UV light.

Why is ONB unsatisfactory?

A．It breaks down when it absorbs NIR light.

B．It falls off the polymer and triggers drug release.

C．It has not come onto the market up till now.

D．It is not effective enough and could be poisonous.

Which word can be used to complete the following process of changes?
⇨⇨⇨

A．protected

B．formed

C．exposed

D．combined