Một dây đàn hồi có chiều dài ℓ, hai đầu cố định. Sóng dừng trên dây có bước sóng dài nhất là
A.$λ_{max} = ℓ$.
B.$λ_{max} =2 ℓ$.
C.$λ_{max} = ℓ/2$.
D.$λ_{max} =4 ℓ$.

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
Drying flowers is the best way ________ them.
A.by preserving
B.preserved
C.to preserve
D.preserve

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
The policeman explained to us ________ get to the market.
A.how we could
B.how could
C.how could we
D.how

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each unfinished sentence.
His parents never allowed him _______.
A.smoke
B.to smoke
C.smoking
D.smokes

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
John proposed _______ Mary on a fine day at the crowded beach in their hometown.
A.getting married
B.that they married
C.to marry
D.to have married

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
The staff at the health center says that resting will help adults as well as children _________ from the common cold.
A.to recover
B.be recovered
C.has recovered
D.will recover

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.
The president has already instructed our branch manager in New York ______ annual sales figures by the end of December
A.reports
B.reporting
C.to report
D.reported

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
My mother always tells me _______ .
A.not to leave my shoes on the carpet
B.not leave my shoes on the carpet
C.don’t leave my shoes on the carpet
D.not to leaves your shoes on the carpet

Mark the letter A,B,C,orD on your answer sheet to indicate the correct answer to each of the following questions.
My mother advised me ______ an apple every day to stay healthy.
A.eat
B.ate
C.to eat
D.eating

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
To bake fish,_______ butter and place it in a generously greased ________ rectangular pan.
A.it brushes with
B.brush it with
C.brushing it with
D.with its brushes