Question 51:

• The correct answer is C. like as.
  • The phrase "consider something like" means to think of something as being similar to something else.

Question 52:

• The correct answer is A. has existed.
  • The sentence is talking about something that has been happening for a long time, so we use the present perfect tense.

Question 53:

• The correct answer is D. During.
  • The phrase "in prehistoric times" refers to a specific period in the past, so we use "During".

Question 54:

• The correct answer is C. nobody.
  • The sentence is saying that no one knows for sure, so we use "nobody".

Question 55:

• The correct answer is C. hadn't been.
  • The sentence is talking about a past time before the paintings were created, so we use the past perfect tense.

Question 56:

• The correct answer is C. may have been.
  • The sentence is expressing a possibility or speculation about the past, so we use "may have been".

Question 57:

• The correct answer is B. works.
  • The subject is "street art", which is singular, so we use the singular verb form "works".

Question 58:

• The correct answer is B. to.
  • The phrase "communicate to the public" means to convey a message to the public.

Question 59:

• The correct answer is B. a few.
  • The sentence is talking about a small number of walls, so we use "a few".

Question 60:

• The correct answer is A. which.
  • "Which" is used to introduce a relative clause that modifies the noun "the famous John Lennon Wall".

Therefore, the completed text is:

If you ask people what they think of graffiti, many say that they consider it like as vandalism, but some actually appreciate the images or words sprayed on walls and think of it as an art form. Interestingly, wall painting has existed in various forms for about the last 40,000 years. During prehistoric times, humans used to paint images of wild animals or people on the walls and ceilings of caves. There are many theories about the reasons for creating these images, but nobody knows for sure. Evidence shows that they were not just used to decorate the living areas, as most of the caves weren't inhabited at the time when the paintings were created. Many experts believe that cave paintings may have been a way of communicating with others.

Although today street art rarely features in art exhibitions, works of art in public places remain popular with artists all over the world. Whether they use chalk drawings, street sculptures, or graffiti, street artists use the streets as their gallery to communicate to the public more directly. There are even a few walls around the world where graffiti is encouraged. For example, the famous John Lennon Wall in Prague in the Czech Republic, which was filled with graffiti featuring lyrics from the Beatles' songs. It was popular for over two decades until the wall was painted over with a single colour in 2000.