You have discovered a strain of mutant yeast cells that divides normally at 30°C but cannot enter M phase at 37°C. You determine that its mitotic cyclin and mitotic Cdk can form a normal M-Cdk complex at both temperatures. Which of the following temperature-sensitive mutations could NOT be responsible for the behavior at 37°C ? a. inactivation of an enzyme that ubiquitinates M cyclin b. inactivation of a protein kinase that acts on the mitotic Cdk kinase c. a decrease in the levels of a transcriptional regulator required for producing M cyclin d. inactivation of a phosphatase that acts on the mitotic Cdk kinase g