This question is a modifier question. Modifier questions can be quite tricky if you are not familiar with one very important rule applying to the use of modifiers.
First of all, what is a modifier?
A modifier is a phrase that provides more information about the subject. Being a phrase, the modifier will not contain a conjugated verb (in that case it would be a clause). A common way to make a modifier is to start a phrase with a present or past participle.
With present participle: Falling like tufts of wool, the snow came down on Peter's field. With past participle: Encouraged by her initial success, Susan decided to continue playing poker.
If, as in the examples above, the modifier starts the sentence, it will be set off from the main clause by a comma. This is what makes question 29 really look like a modifier question. It just offers you different options for one and the same modifier position. Here it becomes very important to understand why one of those options is obviously correct. This all depends on a simple rule.
The most important modifier rule is that when the modifier comes first, the subject needs to follow.
This means that if we switch around the subject in the example sentences above, the modifier will provide information about the wrong thing or person. Let's do this.
Falling like tufts of wool, Peter looked at the snow coming down on his field. Here we shifted the subject from the snow to Peter. Now it is no longer the snow that is falling like tufts of wool but Peter, which is obviously a bit strange - poor Peter.
We can do the same thing with the second example by switching the subject from Susan to playing poker: Encouraged by her initial success, playing poker became one of Susan's favourite hobbies. In this sentence it is playing poker that is encouraged by her initial success. Again - if you known the rule - this is total balderdash.
Most of the ACT questions about modifiers check your understanding of this rule. So does question 29 above.
Let's have a look at the options. C instantly stands out as being barely conform with correct English, so you can easily eliminate it. But if you don't know the modifier rule, all the remaining options might seem quite fine. But, in reality, the situation is very clear.
The first step to get more clarity is to find the subject, i.e. that about which the modifier gives more information. The subject is 'Santiago'. If you know the most important modifier rule, the solution now becomes very simple: you need to find the modifier that really modifies Santiago. What do the different modifiers tell us about Santiago?
Both A and D tell us that Santiago is a 'project', which is obviously nonsense; Santiago is a person who 'builds on' a project. This means that the only valid option is B. Here Santiago remains a person - someone 'building' on a project.
Modifier questions on the ACT English test will typically offer options where the same phrase is reworded alternately using nouns and participles. If you know this, they are pretty easy to spot. Never forget that all questions have a particular aim. All of them test you on a particular subject of grammar; none of them is random. If you understand the aim, you are already halfway there.
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