Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Ever since humans have inhabited the earth, they have made use of various forms of communication. Generally, this expression of thoughts and feelings has been in the form of oral speech. When there is a language barrier, communication is accomplished through sign language in which motions stand for letters, words and ideas. Tourists, the deaf, and the mute have no resort to this form of expression. Many of these symbols of the whole words are very picturesque and exact and can be used internationally: spelling, however, cannot. Body language transmits ideas or thoughts by certain actions, either intentionally or unintentionally. A wink can be a way of flirting or indicating that the party is only joking. A nod signifies approval, while shaking the head indicates a negative reaction.  Other forms of nonlinguistic language can be found in Braille (a system of raised dots read with fingertips), signal flags, Morse code, and smoke signals. Road maps and picture signs also guide, warn and instruct people.

While verbalization is the most common form of language, other systems and techniques also express human thoughts and feelings.

The word "these" in paragraph1 refers to ______.

A. tourists

B. the deaf and the mute 

C. thoughts and feelings

D. sign language motions

Nếu cả 4 chuỗi thức ăn trong các hệ sinh thái dưới đây đều bị ô nhiễm thuỷ ngân với mức độ ngang nhau, con người trong trường hợp nào sẽ bị nhiễm độc nhiều nhất:

A. Tảo đơn bào → động vật phù du → giáp xác → cá → người.

B. Tảo đơn bào → động vật phù du → cá → người.

C. Tảo đơn bào → thân mềm → cá → người.

D. Tảo đơn bào → cá → người.