110. Most scientists doubt Dr. Richards’ claim to _____ signs of microscopic life on Mars. (a) find(b) finding(c) are found(d) having found (x)

có phải là verb tense?

131. Many theorists contend that constructivism represents a viable model for explaining how

mathematics _____.

(a) learn

(b) is learned (x)

(c) are learned

(d) is to learn

134. Southern Star Energy currently _____ more than 130 oil wells in Texas and Oklahoma.

(a) operates (x)

(b) operating

(c) operations

(d) has operated